OpenGL headers

[Matt Turner]

SVN 1176 had to be reverted because the mingw GL headers are too
old/broken. They are from Mesa 4.0 after all. (From *2001*).

As ridiculous as the situation is, I see a few fixes.

1) Include non broken GL headers in ioquake3 SVN. The precedent is
there, ie SDL, AL
2) Direct MingW32 users to a place to get non broken headers in the
README. (http://libsdl.org/extras/win32/cygwin/opengl-devel.tar.gz).
If you want to compile SDL with DirectX support in Mingw you already
have to download the directx devel package from the same directory as
OpenGL devel.

If I get support for this, the question becomes, what GL headers do we
use? The ones in libsdl.org/extra or from Mesa 3D, or other?

Matt Turner