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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. Differential Revision: https://phabricator.services.mozilla.com/D243457