Bug 784841 - Part 5: Integrate virtualenv into SpiderMonkey configure; r=glandium

Python and virtualenv logic is now contained within an m4 file.
SpiderMonkey now requires the mozilla-central virtualenv environment to
build. If SpiderMonkey configure is invoked by mozilla-central's
configure, the virtualenv will be reused. If SpiderMonkey configure is
invoked in isolation, it will search for the virtualenv in the
mozilla-central root.