Newer macOS provides /usr/lib/pkgconfig/libpcre2-8.pc which is broken
because it says headers are in /usr/include but that directory doesn't
exist. It can only be used to find the library, which only exists on
newer macOS at /usr/lib/libpcre2-8.dylib, so it's also unusable.
So, force usage of the subproject for glib.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5060>
