This makes it easier for googletest users to leverage googletest, instead of
having to plug GTEST_CXXFLAGS into CXXFLAGS manually.
I will be following this up with a more proper fix in src.libnames.mk, as
src.libnames.mk should be automatically adding this directory to
CFLAGS/CXXFLAGS when private libraries are referenced. Not doing so can result
in mismatches between base-provided private library and ports-provided library
headers.