[CMake] Clean up the C++ version flag detection and add final test that is always run
This patch cleans up the logic of the C++ version flag detection code, which had accumulated quite a bit of cruft over time. Moreover, it also adds a new test for basic C++11 compliance which is always run, even if DISABLE_CXX_VERSION_CHECK has been set. I think the original code was supposed to also error out in this case, but I didn't really do so. With this patch, DISABLE_CXX_VERSION_CHECK really only disables testing for the various -std=c++xy flags, but the build system will now always make sure that your compiler runs (at least) in C++11 mode.
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