I wanted to lift this next to the definition of AB_CD, but that
doesn't allow to use it in a backend.mk file, due to the order in
which Makefile, config.mk, rules.mk, and backend.mk are processed.
Therefore, I've put it in a tiny include file, so that it can be used
by a Makefile and a backend.mk file.

This allows the `RecursiveMake` backend to owning defining AB_rCD in
backend.mk files, while not requiring consumers to arrange for AB_rCD
in a sibling Makefile.in file.

Other build backends will need to arrange for AB_rCD themselves: see
following commits.

MozReview-Commit-ID: I7GIzRbCCtf

--HG--
extra : rebase_source : 3277fedb43bc3d8007287c223554a085dae2f198
extra : source : 854c0f43a1f74b4e22aa7638b407580240c90dd5