The macro is currently set to 201510 following P0092R1. In Kona
P0505R0 added constexpr in a few more places, which are only very
minor changes. I don't think we need a new macro, so setting the value
to 201703 seems appropriate.

Hmm, although this assumes nobody would implement the latter feature
before the first. Both libstdc++ and libc++ have already implemented
the P0092R1 changes, and VC++ doesn't seem to define any feature-test
macros.