Bump to c++17

[rotu]

replaces #182

This allows us to use std::filesystem, as the logic in filesystem_helper sniffs the current c++ standard. CMake should fall back gracefully to c++14 if c++17 is not available.

This is a correct solution but not the best one, which would be add a library which wraps filesystem and which we link to instead of imposing compilation constraints on consuming projects

[rotu]

Discussion here: #182
Note:

even if we're targeting CMake 14, setting CMAKE_CXX_STANDARD to a newer version will fall back to the previous supported standard if 17 is unavailable.

• experimental/filesystem is not part of either C++14 nor C++17, and in a compiler that implements C++17 should not be used.
• std::filesystem is not part of C++14.

[clalancette]

Obsoleted by #212, so closing.