Need to run all tests in all of C++11, C++14, C++17

Some of our code behaves differently with differing C++ standard version - including potentially some code with explicit `#if __cplusplus` conditions.

We should therefore make sure and have all unit tests run for both C++11, C++14 and C++17. While that will treble compilation time and test execution time, it's still probably a good idea.

See also issue #84 .