build: always generate docs at dist time (bgo #700093)

build: always generate docs at dist time (bgo #700093)

dist tarballs shouldn't ever be released without the documentation,
and this allows us to build the tarballs with pre-generated docs
that get installed on the end system, but don't need to be built
there.  So the end system doesn't need gtk-doc installed, only
the dist system does.

https://bugzilla.gnome.org/show_bug.cgi?id=700093