configure.ac: pthread-stubs is not a thing on GNU/kFreeBSD

As mentioned on the xcb mailing list, the platform uses the GLIBC
forwarding mechanism.

https://lists.freedesktop.org/archives/xcb/2016-November/010896.html



Reported-by: Andreas Boll <andreas.boll.dev@gmail.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Andreas Boll <andreas.boll.dev@gmail.com>