Follow-up to r321684 (Don't use libc++ when cross-building for gcc
arches), and handle two more cases where libc++ includes could be
incorrectly enabled, in case the host compiler is clang 5.0.0, and the
target (cross) compiler is gcc 4.2.1.

Noted by: bdrewery
MFC after: 3 days
X-MFC-With: 321684