Add googletest as a submodule and change the build system to support C++, googletest-based unit-tests.

[verkri]

We discussed with Arpad, that from now on, all unit-tests written in googletest will be next to the libraries. This required a slight modification in the obj outputs, e.g. you will see obj/linux/tests/gtests/libiotc/iotc_test.o

Tested with make gtests, make tests.