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... and remove explicit gcc installation (will be installed with g++
automatically)

This turns on optimization which allows the compiler to discover more
problems and omit some more warnings.

While cmake does not complain on build type 'debug', the cmake
documentation always spells it 'Debug', so take this.

There is no need for separate 'googlemock' and 'googletest' builds,
as the 'googlemock' build includes 'googletest' and it's unit tests.