Meson's gtk-doc support currently requires the main SGML/XML file
to be located in the srcdir, so instead of substituting the module
version in that file directly, use a small auxiliary file that's
included via a custom entity.
https://bugzilla.gnome.org/show_bug.cgi?id=783229
We now link to cogl/clutter forks in a private location, so make
sure we set the rpath of executables to point the runtime linker
to the correct location.