Relax std::optional feature check
Relax std::optional feature check

Summary: The correct value for `__cpp_lib_optional` feature macro is `201606L` (https://en.cppreference.com/w/cpp/feature_test). However, it has been reported that `std::optional` is available with `__cpp_lib_optional == 201603L` so relax the check.

Reviewed By: yfeldblum

Differential Revision: D15806026

fbshipit-source-id: 6223f6df7bceb2ca9c0f27f65c731c373ba9795c

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