File reads were used in ai_assert. The function has side effects, so it should not be used in an assert.
Fixed AddGTest.cmake.
There was an issue regarding Xcode project file. The path of libgtest.a was incorrectly set.
The GTest framework has a more active support base, and natively
supports CMake. Introduce it as an external dependency (using CMake's
ExternalProject_Add), replacing cppunit and porting the associated unit
tests.