Set the environment variable __GLX_FORCE_VENDOR_LIBRARY_0 for all of the GLX
tests instead of __GLX_VENDOR_LIBRARY_NAME.
Individual tests can force libGLX to load a vendor library early by calling
glXGetClientString or similar.
Added a separate test program testglxgetprocaddress_genentry to test the GLX
entrypoint generation by itself instead of lumping that in with
testglxgetprocaddress.
Added a variation of testglxgetprocaddress that doesn't pre-load the vendor
library. This forces it to generate a GLX entrypoint at runtime for
glXExampleExtensionFunction.
In testglxgetprocaddress.c, added a call to glXGetClientString so that it will
load the vendor library without having to pre-load it.
