Support IRIX version of OpenGL multi sampling constants.

Although OpenGL multi sampling extensions originated under IRIX, it doesn't
seem to use the standard names for the constants used and uses "SGI" suffix
for them instead of "ARB".

Add support for IRIX versions of GLX_SAMPLE_BUFFERS_ARB and GLX_SAMPLES_ARB
and also check if these constants are defined at all to avoid compilation
errors if they are not.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63578 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775