Fix this build error (in MesaGLUT-7.6.1)...
glut_cmap.c:23:66: error: X11/Xmu/StdCmap.h: No such file or directory

...by not preventing the cflags that pkg-config finds for glut dependencies
(including 'xmu') from being used.

Defining GLUT_CFLAGS before running the pkg-config prevents the
cflags found by pkg-config from being used.

This patch lets GLUT_CFLAGS that configure & pkg-config work
so hard to set actually get used.

Also make sure the generated configs/autoconf defines GLUT_CFLAGS
used in (at least) src/glut/glx/Makefile.

Signed-off-by: Dan Nicholson <dbn.lists@gmail.com>