Optimised Rust compilation is enabled on passing --enable-optimize to
the configure script. This sets the RUSTFLAGS output variable that gets
picked up by the compile targets RSOBJS and RSSRCS and passed to rustc.
r=glandium
--HG--
extra : commitid : 8thSkfLFXSY
extra : rebase_source : 5ec79b76a187bcbb0f09ad374cf9f763f0adb0d7