On automation, this brings Windows configure time on a clobber from 5:30 to 3:10.
Sadly, because make needs to run under intl/icu/host before configuring
intl/icu/target, intl/icu/host needs to be configured independently. Fortunately,
that's not configured for normal windows builds anyways.
Also, having multiple subconfigures sharing the same cache file is dangerously
racy. Fortunately, not a lot do. In fact, only js/src and $_subconfigure_subdir
do, so force the latter (only used for ldap sdk on comm-central) not to
configure in parallel.
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.
All subconfigures are essentially doing it already, so just inverse the process. That would also limit problems with additional subconfigures (all the recent ones had to come with their own config.cache)
We happen to be lucky currently because e.g. build is created by config.status
before we subconfigure in build/clang-plugin. But further changes break that
luck.
