switch to C17

c17 option added in gcc 8: https://gcc.gnu.org/onlinedocs/gcc-8.5.0/gcc/Standards.html released in 2018: https://gcc.gnu.org/releases.html

c17 officially supported in clang 6: https://clang.llvm.org/c_status.html released in 2018: https://releases.llvm.org/

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The list of all changes integrated in C17 can be found here (the Status marked as C17).