glx: Don't hard-code the name "libGL.so.1" in driOpenDriver (v3)

glx: Don't hard-code the name "libGL.so.1" in driOpenDriver (v3)

Add a macro GL_LIB_NAME to hold the filename that configure comes up with
based on the --with-gl-lib-name and --enable-mangling options.

In driOpenDriver, use the GL_LIB_NAME macro instead of hard-coding
"libGL.so.1".

v2: Add an #ifndef/#define for GL_LIB_NAME so that non-autoconf builds will
    work.
v3: Fix the library filename in the Makefile.

Signed-off-by: Kyle Brenneman <kbrenneman@nvidia.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>