[MLton] case sensitivity in lib/opengl

[Mike Thomas]

Hi Stephen.

| When I try to do a make in lib/opengl/mlton, I get the following
| error.
| 
| gcc -c GL_c.c
| GL_c.c:3:18: Gl_h.h: No such file or directory
| 
| Mike, I suspect you don't see the error on Cygwin because of
| case-insensitivity.  I think a simple edit of GL_c.c will fix the
| problem.

Thanks for that - your suggested fix checked in a minute ago.

Cheers

Mike Thomas.