GoogleTest 1.14.0 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.