pkgconfig: don't put -DG_THREADS_MANDATORY into our pkg-config CFLAGS

pkgconfig: don't put -DG_THREADS_MANDATORY into our pkg-config CFLAGS

If we force -DG_THREADS_MANDATORY onto apps, then g_thread_supported()
will always evaluate to TRUE, so the typical thread initialisation
boilerplate code if (!g_thread_supported()) g_thread_init(NULL); will
no longer work, and the threading system not be initialised and us
printing a warning in gst_init. This may be fine in most cases, since
late initialisation is allowed and automatically done in g_type_init()
since GLib 2.23.2, but let's be cautious and only use this define when
compiling GStreamer itself.

See #608398.