Allow to build host gtest libraries in unbundled build.

You need to include external/libcxx and external/libcxxabi in the
unbundled branche's repo manifest, though.

Change-Id: I778f36ef941beb932ef28c02188b8c855435b051
(cherry-picked from commit e826aa4f99cf7d9e57fe9668e299a2808674ede4)