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.
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Expect WhenDynamicCastToTest.AmbiguousCast to fail
This change reapplies the expected failure from 1.10.0.
Ref: https://github.com/google/googletest/issues/2172