Import GoogleTest 1.14.0

GoogleTest 1.14.0 now requires C++14 to build. Change
`googletest.test.inc.mk` to reflect this requirement.

Adjust the build integration logic to handle the new version of
GoogleTest (add/remove headers/sources as needed).

Tighten down warnings via `CXXFLAGS.clang` instead of ignoring all
warnings. Some new warnings snuck in after I did my last round of fix
submissions upstream.

Also address some overlinking added in the previous version import by
removing superfluous libraries.

===============================

Expect WhenDynamicCastToTest.AmbiguousCast to fail

This change reapplies the expected failure from 1.10.0.

Ref: https://github.com/google/googletest/issues/2172

MFC after: 2 weeks
Reviewed by: asomers, emaste
Differential Revision: https://reviews.freebsd.org/D41399

Merge commit '8ef491440fcaec96f899d73e08873426c78583a4' into googletest-v1.14.0-import

googletest: backport GTEST_SKIP to googletest 1.8.1

This commit backports revisions 00938b2b228f3b70d3d9e51f29a1505bdad43f1e and
59f90a338bce2376b540ee239cf4e269bf6d68ad from googletest's master branch to
our included version of googletest, which is based on 1.8.1. It adds the
GTEST_SKIP feature, which is very useful for a project like FreeBSD where
some tests depend on particular system configurations.

Obtained from: github.com/google/googletest
Sponsored by: The FreeBSD Foundation