We want to include the gtk-doc subproject in release
tarballs, using --include-subprojects, but that only
works if we've actually built the subproject. And
enabling gtk-doc for dist builds is problematic -
it tends to break meson dist.
So declare the gtk-doc dependency independent of
-Dgtk_doc, and use --force-fallback-for for it.
a8eae7afaa
