Respect GLAZE_LIBGL environment variable (if FIPS_LIBGL is unset)

Since the LD_PRELOAD mechanism of fips may not work with some
programs, users may want to run fips within glaze instead, (which uses
LD_LIBRARY_PATH instead of LD_PRELOAD). In order to make this
convenient, fips can recognize that glaze has identified the
appropriate libGL.so library by examining the GLAZE_LIBGL environment
variable.

So, if the user has not specifically set the FIPS_LIBGL variable, and
the GLAZE_LIBGL variable is set, use it to find the libGL.so to load.