Bug 1498031 - Merge code paths for running configure between Tup and Make based backends. r=firefox-build-system-reviewers,mshal

This addresses a related issue along the way: a build that results in running
configure would not update the value of self.config_environment (and therefore
self.substs) as seen from the build driver, so out of date values would have
been used.

The changes to Makefile.in and client.mk made exploit the assumption that by
he time anything in the Make build is running, config.status is up to date.
Users running "make" without the benefit of "mach" will need to manually run
configure when necessary in order to take this into account.

Differential Revision: https://phabricator.services.mozilla.com/D8473

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extra : moz-landing-system : lando