Detect if we build from a git subtree
Backported from gtkmm. If the cairomm directory is a subtree, we are building from a git repository, so we take that into account. This fix the incorrect "You must enable maintainer-mode" error.
Backported from gtkmm. If the cairomm directory is a subtree, we are building from a git repository, so we take that into account. This fix the incorrect "You must enable maintainer-mode" error.