...where an assert in a constexpr function merely triggers a -Wc++1y-extensions
(so doesn't stop configure from assuming HAVE_CXX14_CONSTEXPR, but will later
break with --enable-werror when building include/o3tl/typed_flags_set.hxx).  So
stuff back the complex constexpr test function that was swapped out with
6cc54913 "Work around GCC 5.1 ICE."

Change-Id: I04ba1dc08d96f776b331fa9d0cae1703f2796532