Non-exhaustive list of case-sensitive renames:
GDExtension -> GDNative
GDNATIVE -> GDEXTENSION
gdextension -> gdnative
ExtensionExtension -> Extension (for where there was GDNativeExtension)
EXTENSION_EXTENSION -> EXTENSION (for where there was GDNATIVE_EXTENSION)
gdnlib -> gdextension
gdn_interface -> gde_interface
gdni -> gde_interface
This commit changes the platform detection order to detect mingw
compiling for windows (which defines `__GNUC__`).
This commit also wraps the definition around a guard so it can be
overridden via a define at build-time.
