Allow building as C++ for type-correctness checking

This adds an option (off by default) to BUILD_AS_CXX which will treat the .c files as C++, thus invoking stronger type checking.

Tested on both GL and Vulkan on AMD, and diff carefully reviewed to minimize its changes.

Totally fine if this is not in line with project directions, no hurt feelings if it is rejected - just let me know so I can stop rebasing it 😁