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,sergesanspaille

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.

To make them pickable, though, we replace the use of EnumString.subclass
with actual subclassing, which is a little less convenient, but avoids
having to figure out how to make the classes EnumString.subclass creates
pickable.

This caught a mismatch in media/libpng.

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