#18 Initial FreeBSD support; add missing checks for _WIN32

[Alexey Dokuchaev]

Code is mostly OS-agnostic, but Win32 stuff leaks in a few places, particularly in GL-related code: WGL extension prefix is spelled as GLX under Unix/X11.

Also, checks for Unix should not include just linux. Patches are attached. With them, Quake2XP can be built under FreeBSD with both Clang and GCC 4.8. Actually running it might require more work, but this should be a good starting point.

[Alexey Dokuchaev]

Кстати, возможно, что там где я добавил про фряху #if defined(__linux__) || defined(__FreeBSD__) будет уместнее заменить на #ifndef _WIN32; я не изучал подробно этот код.