configure.ac: add -pthread to PTHREAD_LIBS

configure.ac: add -pthread to PTHREAD_LIBS

As described inline - follow what's written in the manual and what works
for all platforms that Mesa supports.

We want to untangle things leaving only -pthread, yet that has a
potential of causing regressions. Thus we'll do it as a follow-up patch.

As a nice side-effect this resolves issues, where the system lacks
libpthread.so, yet the linker does not warn about it and we and up with
unresolved symbols.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101071
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>