GLX_EXT_import_context allows multiple clients to share the same indirect context. If you can't create an indirect context, you're certainly not going to be able to share one. Hide the extension from the server string if we've disabled indirect contexts.
This turns piglit's tests from fail to skip when indirect contexts are disabled. Since GLX_EXT_import_context has been supported in xfree86-derived servers since day 1 (it was included in the initial GLX code drop from SGI), this is now also a hint to the client that indirect contexts are unlikely to work at all.