gtk-doc for autotools has the concept of module version, that is used to define
the module install path and the devhelp2 basename.
Add a `module_version` parameter to gnome.gtkdoc to replicate the same behavior.
Updated the test checking that the install_dir is properly computed (if not
passed), and that the .devhelp2 file has proper name.
https://gitlab.gnome.org/GNOME/gtk-doc/blob/GTK_DOC_1_29/buildsystems/autotools/gtk-doc.make#L269