Define macros needed by 26bbfea.

The code used build fine on my workstation, since Mesa3D has the
required macros since ever. But on Windows gl.h is still limited
to OpenGL 1 (really?).
Define HAVE_BOOLEAN on Windows

Without this define newer versions of jpeg define a type "bool" as char,
while the MinGW headers define it as "unsigned short". Automake should
have detected that, but...
Refactor the Windows build system

The old Window build was tightly bound to Nuwens MinGW distro. Since
Nuwen decided to take the 64 bit way and dropped 32 bit support, we'll
change over to a more generic MinGW build. Use this chance to clean up
things.

PLEASE NOTE: There's no such thing as a "standard MinGW install".
Therefor our buildsystem cann not and will not be generic. It's still
desinged along the pathes of our build environment. We'll upload it
to the projects FTP server.

Things do consider:
- Dependencies (jpeg, libogg, libvorbis, OpenAL, SDL2, zlib) should be
  installed under /custom/.
- We're now relying on sdl2-config, to be found at /custom/bin. It's
  just a simple shell script, depending on your compiler unsupported
  CFLAGS or LDFLAGS must be removed from it.
- Windows binaries are now stripped. There's no need to supply debug
  symbols on Windows. This reduces quake.exe from ~8.3MB to ~1.6MB.
- The Binaries are still hard linked.
Remove stuff/win32-libs

Those libraries are too old and not needed anymore. Better use those
that are part of the build environment.
Rename VERSION to YQ2VERSION to prevent collisions

Newer jpeg versions (I guess starting with 9) define an macro
"VERSION", colliding with ours. While wie could #undef it, take
the less hacky route and rename it.