stdenv/setup.sh: don't skip post-hooks (close #12032)

So far if no configure script is found or no makefile,
the rest of the phase is skipped, *including* post-hooks.
I find that behavior unexpected/unintuitive.

Earlier version of this patch had problems due to me assuming
that $configureScript is always a simple path, but that turned out
to be false in many cases, e.g. perl.