[1.6.x] Allowed overriding variables in docs/Makefile.
[1.6.x] Allowed overriding variables in docs/Makefile.

Backport of c694e6220e from master

django-nonrel/django,redhat-openstack/django,felixjimenez/django,django-nonrel/django,redhat-openstack/django,dex4er/django,redhat-openstack/django,felixjimenez/django,felixjimenez/django,felixjimenez/django,django-nonrel/django,redhat-openstack/django,django-nonrel/django,dex4er/django,dex4er/django