MAMEdev: Keeping up with C++

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Starting with the December 2020 development cycle (after the anticipated release of MAME 0.227), we will switch to compiling MAME as C++17. Compiler and standard library support for C++17 is now sufficiently widespread that we can update and benefit from the new functionality. The minimum compiler versions and corresponding standard library versions with adequate C++17 support are GCC/libstdc++ 7, clang/llvm/libc++ 6, and Visual C++/MSVCPRT 19.14 (Visual Studio 2017 15.7). This will be an increase in the required clang/llvm/libc++ version.