Because we only ever run one subconfigure, the machinery to execute
several is not useful anymore. Inlining it allows to simplify the code
too, because it doesn't need to be generic anymore. This also removes
the last remaining bits of acwinpaths.m4.

Also remove now unused support for --list in build/subconfigure.py.

Depends on D16380

Differential Revision: https://phabricator.services.mozilla.com/D16381

--HG--
extra : moz-landing-system : lando