Only support XDG directory path configuration on Linux

This also harmonizes the server code to be consistent with Linux desktop
(with warnings printed for relative XDG paths).

Co-authored-by: Rémi Verschelde <rverschelde@gmail.com>
(cherry picked from commit 06c266f231)

doc/classes/OS.xml

@@ -143,7 +143,7 @@
 		<method name="get_cache_dir" qualifiers="const">
 			<return type="String" />
 			<description>
-				Returns the [i]global[/i] cache data directory according to the operating system's standards. On desktop platforms, this path can be overridden by setting the [code]XDG_CACHE_HOME[/code] environment variable before starting the project. See [url=$DOCS_URL/tutorials/io/data_paths.html]File paths in Godot projects[/url] in the documentation for more information. See also [method get_config_dir] and [method get_data_dir].
+				Returns the [i]global[/i] cache data directory according to the operating system's standards. On Linux, this path can be overridden by setting the [code]XDG_CACHE_HOME[/code] environment variable before starting the project. See [url=$DOCS_URL/tutorials/io/data_paths.html]File paths in Godot projects[/url] in the documentation for more information. See also [method get_config_dir] and [method get_data_dir].
 				Not to be confused with [method get_user_data_dir], which returns the [i]project-specific[/i] user data path.
 			</description>
 		</method>
@@ -171,7 +171,7 @@
 		<method name="get_config_dir" qualifiers="const">
 			<return type="String" />
 			<description>
-				Returns the [i]global[/i] user configuration directory according to the operating system's standards. On desktop platforms, this path can be overridden by setting the [code]XDG_CONFIG_HOME[/code] environment variable before starting the project. See [url=$DOCS_URL/tutorials/io/data_paths.html]File paths in Godot projects[/url] in the documentation for more information. See also [method get_cache_dir] and [method get_data_dir].
+				Returns the [i]global[/i] user configuration directory according to the operating system's standards. On Linux, this path can be overridden by setting the [code]XDG_CONFIG_HOME[/code] environment variable before starting the project. See [url=$DOCS_URL/tutorials/io/data_paths.html]File paths in Godot projects[/url] in the documentation for more information. See also [method get_cache_dir] and [method get_data_dir].
 				Not to be confused with [method get_user_data_dir], which returns the [i]project-specific[/i] user data path.
 			</description>
 		</method>
@@ -192,7 +192,7 @@
 		<method name="get_data_dir" qualifiers="const">
 			<return type="String" />
 			<description>
-				Returns the [i]global[/i] user data directory according to the operating system's standards. On desktop platforms, this path can be overridden by setting the [code]XDG_DATA_HOME[/code] environment variable before starting the project. See [url=$DOCS_URL/tutorials/io/data_paths.html]File paths in Godot projects[/url] in the documentation for more information. See also [method get_cache_dir] and [method get_config_dir].
+				Returns the [i]global[/i] user data directory according to the operating system's standards. On Linux, this path can be overridden by setting the [code]XDG_DATA_HOME[/code] environment variable before starting the project. See [url=$DOCS_URL/tutorials/io/data_paths.html]File paths in Godot projects[/url] in the documentation for more information. See also [method get_cache_dir] and [method get_config_dir].
 				Not to be confused with [method get_user_data_dir], which returns the [i]project-specific[/i] user data path.
 			</description>
 		</method>

platform/osx/os_osx.mm

@@ -2285,14 +2285,6 @@ MainLoop *OS_OSX::get_main_loop() const {
 }
 
 String OS_OSX::get_config_path() const {
-	// The XDG Base Directory specification technically only applies on Linux/*BSD, but it doesn't hurt to support it on macOS as well.
-	if (has_environment("XDG_CONFIG_HOME")) {
-		if (get_environment("XDG_CONFIG_HOME").is_abs_path()) {
-			return get_environment("XDG_CONFIG_HOME");
-		} else {
-			WARN_PRINT_ONCE("`XDG_CONFIG_HOME` is a relative path. Ignoring its value and falling back to `$HOME/Library/Application Support` or `.` per the XDG Base Directory specification.");
-		}
-	}
 	if (has_environment("HOME")) {
 		return get_environment("HOME").plus_file("Library/Application Support");
 	}
@@ -2300,26 +2292,10 @@ String OS_OSX::get_config_path() const {
 }
 
 String OS_OSX::get_data_path() const {
-	// The XDG Base Directory specification technically only applies on Linux/*BSD, but it doesn't hurt to support it on macOS as well.
-	if (has_environment("XDG_DATA_HOME")) {
-		if (get_environment("XDG_DATA_HOME").is_abs_path()) {
-			return get_environment("XDG_DATA_HOME");
-		} else {
-			WARN_PRINT_ONCE("`XDG_DATA_HOME` is a relative path. Ignoring its value and falling back to `get_config_path()` per the XDG Base Directory specification.");
-		}
-	}
 	return get_config_path();
 }
 
 String OS_OSX::get_cache_path() const {
-	// The XDG Base Directory specification technically only applies on Linux/*BSD, but it doesn't hurt to support it on macOS as well.
-	if (has_environment("XDG_CACHE_HOME")) {
-		if (get_environment("XDG_CACHE_HOME").is_abs_path()) {
-			return get_environment("XDG_CACHE_HOME");
-		} else {
-			WARN_PRINT_ONCE("`XDG_CACHE_HOME` is a relative path. Ignoring its value and falling back to `$HOME/Library/Caches` or `get_config_path()` per the XDG Base Directory specification.");
-		}
-	}
 	if (has_environment("HOME")) {
 		return get_environment("HOME").plus_file("Library/Caches");
 	}

platform/server/os_server.cpp

@@ -207,34 +207,55 @@ void OS_Server::run() {
 }
 
 String OS_Server::get_config_path() const {
+#ifndef __APPLE__
 	if (has_environment("XDG_CONFIG_HOME")) {
+		if (get_environment("XDG_CONFIG_HOME").is_abs_path()) {
 			return get_environment("XDG_CONFIG_HOME");
-	} else if (has_environment("HOME")) {
-		return get_environment("HOME").plus_file(".config");
 		} else {
-		return ".";
+			WARN_PRINT_ONCE("`XDG_CONFIG_HOME` is a relative path. Ignoring its value and falling back to `$HOME/.config` or `.` per the XDG Base Directory specification.");
 		}
 	}
+#endif
+
+	if (has_environment("HOME")) {
+		return get_environment("HOME").plus_file(".config");
+	}
+	return ".";
+}
 
 String OS_Server::get_data_path() const {
+#ifndef __APPLE__
 	if (has_environment("XDG_DATA_HOME")) {
+		if (get_environment("XDG_DATA_HOME").is_abs_path()) {
 			return get_environment("XDG_DATA_HOME");
-	} else if (has_environment("HOME")) {
-		return get_environment("HOME").plus_file(".local/share");
 		} else {
-		return get_config_path();
+			WARN_PRINT_ONCE("`XDG_DATA_HOME` is a relative path. Ignoring its value and falling back to `$HOME/.local/share` or `get_config_path()` per the XDG Base Directory specification.");
 		}
 	}
+#endif
+
+	if (has_environment("HOME")) {
+		return get_environment("HOME").plus_file(".local/share");
+	}
+	return get_config_path();
+}
 
 String OS_Server::get_cache_path() const {
+#ifndef __APPLE__
 	if (has_environment("XDG_CACHE_HOME")) {
+		if (get_environment("XDG_CACHE_HOME").is_abs_path()) {
 			return get_environment("XDG_CACHE_HOME");
-	} else if (has_environment("HOME")) {
-		return get_environment("HOME").plus_file(".cache");
 		} else {
-		return get_config_path();
+			WARN_PRINT_ONCE("`XDG_CACHE_HOME` is a relative path. Ignoring its value and falling back to `$HOME/.cache` or `get_config_path()` per the XDG Base Directory specification.");
 		}
 	}
+#endif
+
+	if (has_environment("HOME")) {
+		return get_environment("HOME").plus_file(".cache");
+	}
+	return get_config_path();
+}
 
 String OS_Server::get_system_dir(SystemDir p_dir, bool p_shared_storage) const {
 	String xdgparam;

platform/windows/os_windows.cpp

@@ -3604,14 +3604,6 @@ uint64_t OS_Windows::get_embedded_pck_offset() const {
 }
 
 String OS_Windows::get_config_path() const {
-	// The XDG Base Directory specification technically only applies on Linux/*BSD, but it doesn't hurt to support it on Windows as well.
-	if (has_environment("XDG_CONFIG_HOME")) {
-		if (get_environment("XDG_CONFIG_HOME").is_abs_path()) {
-			return get_environment("XDG_CONFIG_HOME").replace("\\", "/");
-		} else {
-			WARN_PRINT_ONCE("`XDG_CONFIG_HOME` is a relative path. Ignoring its value and falling back to `%APPDATA%` or `.` per the XDG Base Directory specification.");
-		}
-	}
 	if (has_environment("APPDATA")) {
 		return get_environment("APPDATA").replace("\\", "/");
 	}
@@ -3619,29 +3611,13 @@ String OS_Windows::get_config_path() const {
 }
 
 String OS_Windows::get_data_path() const {
-	// The XDG Base Directory specification technically only applies on Linux/*BSD, but it doesn't hurt to support it on Windows as well.
-	if (has_environment("XDG_DATA_HOME")) {
-		if (get_environment("XDG_DATA_HOME").is_abs_path()) {
-			return get_environment("XDG_DATA_HOME").replace("\\", "/");
-		} else {
-			WARN_PRINT_ONCE("`XDG_DATA_HOME` is a relative path. Ignoring its value and falling back to `get_config_path()` per the XDG Base Directory specification.");
-		}
-	}
 	return get_config_path();
 }
 
 String OS_Windows::get_cache_path() const {
 	static String cache_path_cache;
 	if (cache_path_cache == String()) {
-		// The XDG Base Directory specification technically only applies on Linux/*BSD, but it doesn't hurt to support it on Windows as well.
+		if (has_environment("LOCALAPPDATA")) {
-		if (has_environment("XDG_CACHE_HOME")) {
-			if (get_environment("XDG_CACHE_HOME").is_abs_path()) {
-				cache_path_cache = get_environment("XDG_CACHE_HOME").replace("\\", "/");
-			} else {
-				WARN_PRINT_ONCE("`XDG_CACHE_HOME` is a relative path. Ignoring its value and falling back to `%LOCALAPPDATA%\\cache`, `%TEMP%` or `get_config_path()` per the XDG Base Directory specification.");
-			}
-		}
-		if (cache_path_cache == String() && has_environment("LOCALAPPDATA")) {
 			cache_path_cache = get_environment("LOCALAPPDATA").replace("\\", "/");
 		}
 		if (cache_path_cache == String() && has_environment("TEMP")) {