[Nel] Pb with glx version

[Vincent Caron]

I'm having the same trouble as Bernard Hugueney had. I'm using Debian Sid
(unstable), and the nel/ configure script complains about OpenGL version
being too old (GL_GLEXT_VERSION < 7). Debian packages the original XFree 4.0.2
header files, XFree says 4.0.2 uses Mesa 3.4, and if you downloa Mesa 3.4 you'll
see that GL_GLEXT_VERSION is set to 6.

Since the latest version from SGI's OpenGL sample implementation is set to 7,
and Mesa's CVS stick to that 6, I guess Nevrax developpers use somewhat
cstomized RPMs (let me guess ... Mandrake ? :))

I suggest that the configure script should be relying on version 6 for now,
in order we claim what's really implemented by XFree 4.0.2.