configure.ac: Ensure that we handle case of non-present --target

@rwbarton pointed out that this could be an issue during review, but I
assumed from my point sample of three test builds that this wouldn't be
necessary.  Sadly, none of these builds were on Windows, which indeed
does fail.

Strangely, only Simon and Harbormaster have been able to replicate the
issue (which apparently manifests as libffi thinking it's building for
unix).  I've been completely unable to replicate the failure in my own
builds, neither locally nor on the Harbormaster machine.

Test Plan: Validate on Windows

Reviewers: austin, Phyx, hvr

Reviewed By: Phyx

Subscribers: thomie, rwbarton, erikd

Differential Revision: https://phabricator.haskell.org/D3304