New build system based on Q2PRO

 - Linux GLX builds only for now
 - No bundled static libraries used
 - Some WITH_* defines might be missing
Import minimal SDL2 GLimp, Q2PRO again
Initial system split POSIX/Windows
Import SDL2 sound backend from Q2PRO
Update .gitignore for new build system
Use new versioning
Legacy sound code purge
Win32 support returned with SDL2

Some stuff were stubbed out for later SDL2 implementation, some
were completely removed as the have been replaced by SDL2.

No cleanups were done. Building requires some .config options
with mingw and the "official" release .config generation script
will probably be imported to the repository later.
Remove old build scripts and project files

Building is now streamlined into main Makefile and it is possible
to build both Linux and Windows binaries on Linux without much
hassle. OSX building will be fixed later and the osx/ directory
will be removed after sys_osx has been migrated.
Another big purge of platform specific code
Add icon to SDL window
Fix bug with sizeof usage with struct
Forgot to include in_sdl2.c
Fix big chunk of warnings
Don't restart sound system on s_khz change if not required
Remove Xpm dependency as we now use raw image data for icon
Enable zip support, missing define
Targets are system and CPU specific
Enable in_mouse to select between normal and raw
minizip is now part of the build for convenience