Meson is on track to replace autotools as the build system of choice,
so support it in addition to autotools. If all goes well, we'll
eventually be able to drop the latter ...
https://bugzilla.gnome.org/show_bug.cgi?id=783229
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
