stdenv: accept wider range of $configureScript options

Fixes #12632.

I think it's better to quote this variable in general, because it is
common and even documented to pass space-separated commands in there.
The greps should just fail in that case and `if` won't proceed
which seems fine for such cases, and it's certainly better than
passing additional unintended parameters to grep
(which was happening all the time before).