They need unix on non-windows, for Utility.Env, which Build.Configure uses,
but cabal can't express that in a custom-setup stanza.
To avoid this problem, Utility.Env would need to be moved into
unix-compat..
Windows needs the setenv package in custom-setup, but I don't want to
pull it in on unix, which would probably break some builds and need more
work. Instead, split out setEnv to a separate module.
Quite likely, unix-compat will get a portable environment layer, and
then both modules can be removed from here.
This commit was sponsored by Øyvind Andersen Holm.
Didn't know that this library existed!
This includes making git-annex not re-exec itself on start on windows, and
making the test suite on Windows run tests without forking.
