Bug 968245 - Regenerate mozinfo.json as part of build backend; r=glandium

Previously, mozinfo.json was only generated as configure time.
Unfortunately, the build dependencies did not capture this relationship.
So, changes to mozinfo.py (or any supporting Python file) would not
trigger mozinfo regeneration, possibly leading to clobbers.

This patch moves mozinfo.json generation from the body of config.status
to the build backend. We had to add an AC_SUBST so the build config
knows when to build mozinfo.json. This was needed because js/src's build
system doesn't define all the required variables to create mozinfo.json.
Once js/src's configure/config.status is merged into the main build
config tree, this workaround can be removed.

While we were here, mozinfo.json was made to have consistent output and
its changes are now viewable with config.status --diff.

--HG--
extra : rebase_source : e91ed7173efdcde5831ae13b1ce69fc3cf32af97
extra : amend_source : 1610bcc6c3af764fdf685ee76188fb211bd828de