Before, we would run configure in both top-level and js/src, and both
configures would traverse their own set of moz.builds, without actual
knowledge about the other. With this change, both configures still run,
but only top-level traverses moz.build files, and uses js/src's
config.status when traversing its moz.build files. This allows a better
sharing of information between both build systems and the removal of many
hacks.

This also moves running libffi and icu configure to top-level.

Standalone js builds still have their own configure doing moz.build traversal,
as before.

--HG--
rename : config/autoconf.mk.in => config/autoconf-js.mk.in
rename : config/emptyvars.mk.in => config/emptyvars-js.mk.in