auto merge of #1292 : metajack/servo/not-parallel, r=larsbergstrom

Sub-builds are still parallelized. This is a workaround for rustpkg
issues. See https://github.com/mozilla/rust/issues/9650
Implement :root pseudo-class. Fix #1263, fix #1265
auto merge of #1293 : SimonSapin/servo/root, r=SimonSapin

Rebase/squash of @therealglazou’s #1265
Move backup related stuff to its own Makefile.

This will prevent Makefile errors from affecting backups. This happens when
different branches include Makefile helpers other branches don't have. This
should be more reliable.