Chia-I Wu authored Jul 17, 2009 and Brian Paul committed Jul 17, 2009

This commit introduces a "current" system to manage per-thread info.  It
uses TLS, if GLX_USE_TLS is defined, or pthread, if PTHREADS is defined.
If none of them are defined, it uses a dummy implementation that is just
like before.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>