Bug 978594 - Part 2: Add support for expressing per-source build flags to moz.build; r=glandium

--HG--
extra : rebase_source : 254d8acf1a6a53845a2e2d6d895217515ea7b836
Bug 978594 - Part 3: Port some of the per-source flags to moz.build; r=glandium

--HG--
extra : rebase_source : fe4cd059eddda221af420e1517250772816d7ee8