Import patch from libc++ r197313 which allows using libc++ headers with gcc

Differential Revision:	https://reviews.freebsd.org/D942
Reviewed by:	imp


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273066 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f