Make the build system automatically build the C# Api assemblies to be shipped with the editor.
Make the editor, editor player and debug export templates use Api assemblies built with debug symbols.
Always run MSBuild to build the editor tools and Api assemblies when building Godot.
Several bugs fixed related to assembly hot reloading and restoring state.
Fix StringExtensions internal calls not being registered correctly, resulting in MissingMethodException.
It's the recommended way to set those, and is more portable
(automatically prepends -D for GCC/Clang and /D for MSVC).
We still use CPPFLAGS for some pre-processor flags which are not
defines.
This has already been fixed in Mono both master and 2019-06 (no other branch other than the skipped 2019-04 branch uses pthread_mutexattr_setprotocol).
Include paths are processed from left to right, so we use Prepend to
ensure that paths to bundled thirdparty files will have precedence over
system paths (e.g. `/usr/include` should have lowest priority).
