Later versions of Mesa are needed to get advanced OpenGL
functionality, namely OpenGL 2.0.

Mesa 7.5 includes some of my work to get Mesa to build easily on
the platforms we care about.  We now use Mesa's "install" target,
instead of trying to roll our own.

git-svn-id: http://visit.ilight.com/svn/visit/trunk/src@7999 18c085ea-50e0-402c-830e-de6fd14e8384