Allow the user to specify channel bits of 16 or 32 to enable OSMesa16 or
OSMesa32 instead of the default OSMesa. This option is controlled
through the option --with-osmesa-bits=BITS and is only honored when the
driver is osmesa.

The osdemos are not enabled in the 16 or 32 bit case because the
Makefile is currently hardcoded to link to -lOSMesa.