Add limited, experimental support for MinGW and Clang

[alemariusnexus]

This adds some basic support for building BlackBone with MinGW and Clang. See the new file README_MINGW_CLANG.md for the current limitations.

Most of the changes are to fix compiler errors or warnings. It was also necessary to include the ATL implementation from ReactOS (LGPL), because the original ATL only works with MSVC. This 3rd-party ATL is only used when building with MinGW/Clang.