Allow osmesa to be enabled or disabled

Allow osmesa to be enabled or disabled

The conditional in src/mesa/Makefile currently hardcodes the cases where
libOSMesa can be built on libGL. Likewise, the xlib case always includes
libOSMesa in the stand-alone target.

This changes the conditional to a loop over the DRIVER_DIRS variable.
This means that any driver configuration can enable or disable osmesa.
The current "stand-alone" rule is changed so that DRIVER_DIRS=x11 and
DRIVER_DIRS="x11 osmesa" are both respected.

The configure option is changed to --enable-gl-osmesa as this change
allows libOSMesa to be built upon any of the libGL-enabling drivers.