automake: Honor GL_LIB for mangled/custom lib names

automake: Honor GL_LIB for mangled/custom lib names

Commit 2d4b77c7 (automake: Convert src/mesa/drivers/x11/Makefile to
automake, 2012-06-12) dropped the old Makefile, which used GL_LIB, and
replaced it with a Makefile.am hard-coding the name "GL".  This broke
handling of --enable-mangling and --with-gl-lib-name options which
depend on GL_LIB to specify the GL library name.

Use "@GL_LIB@" in src/mesa/drivers/x11/Makefile.am to configure the
library name.  Also use this approach to simplify src/glx/Makefile.am
and drop the HAVE_MANGLED_GL conditional.  While at it, fix the
compatibility link we create in "lib" for the software-only driver to
use version GL_MAJOR instead of hard-coding "1".

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