configure.ac: bring back pthread-stubs check

Accidentally removed with earlier commit. Although rather than simply
adding the check for all platforms, use the same heuristics as we do in
mesa.

Namely: pthread-stubs is a thing only for BSD platforms. Everyone else
should have lightweight pthread API in their C runtime library.

Fixes: 4ecd1ef0 ("xf86drm: fix null termination of string buffer")
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>