[SCons] Add GDExtension method to scons.

[Faless]

Called like:

env.GDExtension("my_library_name", sources)

Optional parameters:

• entry_symbol = "" ({name}_library_init if empty)
• output_dir = "" (output extension folder, {name} if empty)
• library_dir = "lib" (library subdir, relative to the output_dir)

Builds:

{output_dir}/
  {name}.gdextension
  {library_dir}/
    lib{name}.{platform-arch-library-suffix}

Draft:

• Optional override for for .gdextension file (copy instead of generate).
• Extension library auto detect feature in Godot (cc @groud )
• Handle macos framework and their custom structure/Info.plist (auto-generate with optional override? Have it handled by godot platform exporter? cc @bruvzg )

[bruvzg]

Handle macos framework and their custom structure/Info.plist (auto-generate with optional override? Have it handled by godot platform exporter?

Exporter will copy the whole directory, without checking internal structure.

Info.plist needs only library file name (always same as framework name, but without extension), id (which should be overridable by user), and (optionally) readable name. See https://github.com/godotengine/godot/blob/2b505b74b9b0a7005586ecaa9aa1236e86b18437/modules/text_server_adv/gdextension_build/SConstruct#L650-L665