Small mingw compile fixes
Added clipboard support on linux using X11.

We're still using SDL1.2, but now some functions use X11, so if we every fully switch to X11, we can keep at least some of the code.

Courtesy of http://www.libsdl.org/projects/scrap/, which was gratefully released into the public domain.
Fixes windows headers to use the exact mingw capitalization.

This is necessary for cross-compilation on linux, as linux treats different capitalization as different files.
Removed a huge unnecessary MinGW hack.

See Toolchain-mingw32.cmake for a better solution.