doc: Set DOC_MODULE_VERSION
doc: Set DOC_MODULE_VERSION

This will avoid file collisions with future API versions and makes
different API versions completely parallel installable.

Based on similar patch to libchamplain from Marvin Schmidt
<marv@exherbo.org>.

https://bugzilla.gnome.org/show_bug.cgi?id=707928

mlundblad/geocode-glib,Distrotech/geocode-glib,mlundblad/geocode-glib,mlundblad/geocode-glib,Distrotech/geocode-glib