Right now the mozconfig file gets sourced by both configure and make for 'export' statements. Another option is to have configure save the exported variables in a makefile for make to use, so that any environment changes must go through configure. See bug 903118 for some context.
Thank you for filing the followup! As part of this, we should also look into having the build tool remove all unallowed environment variables from the environment before kicking off the build backend.
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