OSMesa support

I'm opening this pull request more to start a dialog about how to do this properly than to submit these changes, which are just a hack that has been useful for my purposes.

Mostly this is quick, "do nothing" ifdefs when built with
SYSTEM=linux-osmesa, to support the off-screen rendering mode of Mesa in
headless tests.  Support in glewinfo and visualinfo is enough to get
them compile, but insufficient for them to run successfully.

I've been using this for headless testing of a project using glew for almost a year now.