Bug 1840537 - Enable value checking in moz.build for CONFIG variables derived...

Bug 1840537 - Enable value checking in moz.build for CONFIG variables derived from target.*. r=firefox-build-system-reviewers,andi

This makes the types from mozbuild.configure.constants repr()-able, and
repr()-ed in config.status, which, when processing moz.build, translates
into value checking for comparison tests involving the variables.

This caught a mismatch in media/libpng.

Differential Revision: https://phabricator.services.mozilla.com/D182141