Implemented cmake build for IDE.

Only on windows and linux so far, not yet tested on windows.

It also adds a new CMake/PolycodeDependencies.cmake that does most of the work of finding and linking polycode's dependencies for you. New cmake projects can just use this, and if something changes about polycode dependencies, we need only update one file.
Fixed a bug in the IDE cmake file.

Used copy instead of copy_directory to copy the Standalone/ dir.
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.
Merge pull request #335 from CIB/mingw

MinGW fix for linux cross-compilation