Remove the ugly switch in ctx/optional.hpp
There's a switch that go include the "right" optional library file (in or in <experimental/optional>) and then jam it into a wrapper namespace for usage. Just remove this ugly macro switch once we switch the -std=c++1z to -std=c++17. cf. : https://gitlab.lrde.epita.fr/mroynard/metaprog/blob/dev/include/metaprog/ctx/optional.hpp#L6