A previous iteration referenced/used googletest.test.flags.mk, not
googletest.test.inc.mk. The latter name is what I settled on as this
Makefile snippet can include more logic than flags.
Long story short, some of the tests were failing because they expected either
dynamic_cast or RTTI to be functional and it wasn't.
Move all common CXXFLAGS out to googletest.test.inc.mk and reference it from
googletest.test.mk and .../googletest/Makefile.inc
googletest.test.mk is a rudimentary wrapper around the plain test interface
(for now), which only supports C++ programs, specified by the `GTESTS`
variable.
In the future, kyua will support gtests in a more native manner.