In simple terms, add_subdirectory() is replaced with ExternalProject_Add().
This change is inspired by https://crascit.com/2015/07/25/cmake-gtest/
with one difference, no download, using the source we already have.
Before this change, make debug-test must be preceded by make clean.
Otherwise, a subsequent build would be polluted by cmake options made
by tests/gtest/.
Also removed the changed compiler flags. My test build did not have
the affected warnings.
32d7d04858