gnome-doc-utils: ensure a native variant exists for document generation
gnome-doc-utils-native must be available to do the build time generation of
documentation however most gnome projects will do a pkgconfig check for the
host version of gnome-doc-utils.
This patch adds gnome-doc-utils-native and makes gnome-doc-utils depend on it
so that recipe writers can just add gnome-doc-utils to their DEPENDS and the
system will do the right thing.
