Log message:
MesaLib18: adjust pthread stubs patch to ensure no OS picks it up, as we
don't provide libpthread stubs.

Note that the name is misleading, libpthread stubs refers to a library
that only pulls in libpthread if a set of functions is not available as
stubs as libc. It is only a pkgconfig file that answers the question
"pull in libpthread? Y/N".

Replace non-existent libpthread stubs buildlink3.mk with the pthread
buildlink3.mk, we should be linking with libpthread for most (all?)
OSes.

Log message:
MesaLib18: remove workaround for pkgsrc GCC already fixed in GCC (upstream
and in pkgsrc)

Log message:
graphics/MesaLib18: import MesaLib-18.2.0 from wip/MesaLib

MesaLib is a 3-D graphics library with an API which is very similar to
that of OpenGL*.  To the extent that Mesa utilizes the OpenGL command syntax
or state machine, it is being used with authorization from Silicon Graphics,
Inc.  However, the author makes no claim that Mesa is in any way a
compatible replacement for OpenGL or associated with Silicon Graphics, Inc.