SCons: Add method to detect Emscripten and use it for warnings config

Emscripten is LLVM-based so we want to follow the same logic. But we can't just
put it as a match in `methods.using_clang()` as that would mess with the
compiler version detection logic used to restrict old GCC and Clang releases.

SConstruct

@@ -510,7 +510,7 @@ if selected_platform in platform_list:
 
         if methods.using_gcc(env):
             common_warnings += ["-Wshadow-local", "-Wno-misleading-indentation"]
-        elif methods.using_clang(env):
+        elif methods.using_clang(env) or methods.using_emcc(env):
             # We often implement `operator<` for structs of pointers as a requirement
             # for putting them in `Set` or `Map`. We don't mind about unreliable ordering.
             common_warnings += ["-Wno-ordered-compare-function-pointers"]
@@ -532,7 +532,7 @@ if selected_platform in platform_list:
                 env.Append(CXXFLAGS=["-Wplacement-new=1"])
                 if cc_version_major >= 9:
                     env.Append(CCFLAGS=["-Wattribute-alias=2"])
-            elif methods.using_clang(env):
+            elif methods.using_clang(env) or methods.using_emcc(env):
                 env.Append(CCFLAGS=["-Wimplicit-fallthrough"])
         elif env["warnings"] == "all":
             env.Append(CCFLAGS=["-Wall"] + common_warnings)
@@ -548,7 +548,7 @@ if selected_platform in platform_list:
                 env.Append(CXXFLAGS=["-Wno-error=cpp"])
                 if cc_version_major == 7:  # Bogus warning fixed in 8+.
                     env.Append(CCFLAGS=["-Wno-error=strict-overflow"])
-            elif methods.using_clang(env):
+            elif methods.using_clang(env) or methods.using_emcc(env):
                 env.Append(CXXFLAGS=["-Wno-error=#warnings"])
         else:  # always enable those errors
             env.Append(CCFLAGS=["-Werror=return-type"])

methods.py

@@ -828,6 +828,10 @@ def using_clang(env):
     return "clang" in os.path.basename(env["CC"])
 
 
+def using_emcc(env):
+    return "emcc" in os.path.basename(env["CC"])
+
+
 def show_progress(env):
     import sys
     from SCons.Script import Progress, Command, AlwaysBuild