autoconf: Support Motif widgets in GLw with --enable-motif

autoconf: Support Motif widgets in GLw with --enable-motif

Add an --enable-motif option, which will enable the Motif widgets in
libGLw and link it with libXm. The Motif installation information will
be gathered from the motif-config script (this comes with LessTif) or
fallback to the standard autoconf checks.

To allow the location of the Motif headers to be set from configure, the
default setting of -I/usr/include/Motif1.2 has been moved into
configs/default and then passed to the Makefile through the MOTIF_CFLAGS
variable.