Increase C++ Standard to c++17
The current minimal version of gcc (7.3) and clang (9.0) support c++17 (with the only exception of Matching template template parameters to compatible arguments (p0522r0) in clang).
The current minimal version of gcc (7.3) and clang (9.0) support c++17 (with the only exception of Matching template template parameters to compatible arguments (p0522r0) in clang).