We know that *-apple-* is leading_underscores, and .dylib.
It is also better to test for TargetVendor being apple, rather than
relying on targetOS, which could be macOS, iOS, tvOS, watchOS,
or any other glorious name apple could come up with.

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, erikd

Differential Revision: https://phabricator.haskell.org/D3557