This is done in the most failsafe way by making sure that all variables
that where previously set through MOZ_CREATE_CONFIG_STATUS() from
old-configure are still set from init.configure.
Some of them are probably not used by the build system but their removal
is left to another patch, as this excision is already a big change in
itself.
As a side effect, remove all m4-related configuration and usage.
This impacts all mozconfig files that used to just set a variable to be
passed to the configuration step, instead of exporting them. As this
method was relying on old-configure, we now require to export the
variables to be passed to the configure step, the others are ignored
(but can still be used as local variables).
Differential Revision: https://phabricator.services.mozilla.com/D243457
7fdcb7d5db