[CMake] Rewrite C++ standard detection test
This patch is a complete rewrite of the existing C++ standard version test that tries the various -std=c++xy flags and tries to figure out which version of the standard to enable. While my cleaned-up version of the old implementation generally works, it has a few problems: - it is not really extensible to newer standards (that will lead to the same kind of mess as before). - More importantly, there is no way to say: I want C++11, but not C++14 (or, in the future, I want C++14, but not C++17). Being able to do so is important for testing and compatibility reason. So this patch mostly starts from scratch: There is a list of supported C++ standards (currently 17,14,11) and corresponding lists of compiler tests and compiler flags to switch the compiler to the respective standards (there might be different values for the switch, e.g. some compilers accept -std=c++1y, but not -std=c++14, and recent versions of GCC reject -std=c++1x). The code then just iterates through these versions and flags until it finds one that works. If that flag / version combo also passes the compile test, the maximum supported standard version is recorded in the variable CXX_MAX_SUPPORTED_STANDARD and the test stops. If the test fails to find a working combination, it issues a warning to the user and only records support for C++03. The test can be influenced by two CMake variables: - DISABLE_CXX_VERSION_CHECK already existed in the old version of the test. It completely disables all testing and requires the user to manually set CXX_MAX_SUPPORTED_STANDARD to the correct value. Moreover, any required compiler command line switches have to be manually added to CMAKE_CXX_FLAGS. - CXX_MAX_STANDARD defines the maximum version of the standard that the build system will try to enable. With this new switch, it becomes possible to limit the compiler to an older version of the standard than what it could theoretically support. For now, this defaults to C++14.
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