build: use libc++ on OSX

V8 now depends on C++11 runtime features. On OSX this requires us to
link against the libc++ library rather than the deprecated default
that is provided with -mmacosx-version-min=10.7.

PR-URL: https://github.com/nodejs/node/pull/8317


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michaël Zasso <targos@protonmail.com>