[MLton] case sensitivity in lib/opengl

[Stephen Weeks]

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.