Fixes the following build error and missing '-pthread' argument when
linking:
subprojects/gstreamer/plugins/tracers/libgstcoretracers.so.p/gstleaks.c.o: In function `gst_leaks_tracer_setup_signals':
/work/build32/../subprojects/gstreamer/plugins/tracers/gstleaks.c:919: undefined reference to `pthread_atfork'
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/571>
This is what Autoconf already does for us, so just do this. Avoids
people making typos while adding header or function checks. Because we
use a config.h.meson, such typos won't even be noticed.
Also, starting from Meson 0.36.0, the XCode 8 workaround that we use for
clock_gettime is no longer needed.