Bug 1046533 - Completely wrap subconfigures. r=mshal

While bug 903369 added some kind of wrapping, msys mangling on Windows made
it hard to make the python wrapper invoke subconfigures itself. This change
overcomes this, allowing to run subconfigures entirely independently of
the main configure if necessary, or to do more fancy checks without having
to resort to m4 and shell.
Bug 1043268 - Skip subconfigures when it makes sense. r=mshal
Bug 1047225 - Properly run the test from bug 442629. r=bz

--HG--
rename : uriloader/exthandler/tests/mailcap => uriloader/exthandler/tests/unit/mailcap