As we move our compile command lines into mozbuild we want to stop using DEFINES
directly. This patch adds a variable that can be set from Makefiles to contribute
defines to a compile command line to help the transition.
MozReview-Commit-ID: 5xLB06pzpJY
--HG--
extra : rebase_source : 7ca0b9d8f5a77ec55eb751f75f58a69e64efb21d