erthink: fix/refine cxx14_constexpr & constexpr_assert for GCC < 6.
erthink: fix/refine cxx14_constexpr & constexpr_assert for GCC < 6.

leo-yuriev/libfptu,leo-yuriev/libfpta,leo-yuriev/libfpta,leo-yuriev/libfptu