[vtkusers] compile problem on linux Redhat 6.2 with Mesa-3.2-2

[Bill Hoffman]

The problem with Mesa and VTK is that at some point Mesa dropped the
#define MESA in its gl.h file.   This was used by VTK to detect mesa
extensions that allowed off screen rendering.   I have checked in
a fix in vtkMesaRenderWindow.h that should fix the problem.  If you don't
want to do an update, the fix is to configure --with-opengl and not 
--with-mesa.   It will mean that off screen rendering will not work, but
normal VTK rendering should work fine.