Don't let old Clang using -std=c++11 mis-detect HAVE_CXX14_CONSTEXPR

...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
6cc5491307 "Work around GCC 5.1 ICE."

Change-Id: I04ba1dc08d96f776b331fa9d0cae1703f2796532

configure.ac

@@ -6347,6 +6347,16 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
     }
     enum E { e };
     auto v = g(E::e);
+
+    struct S {
+        int n_;
+        constexpr bool f() {
+            int n = n_;
+            int i = 0;
+            while (n > 0) { --n; ++i; }
+            return i == 0;
+        }
+    };
     ]])], [cxx14_constexpr=yes], [cxx14_constexpr=no])
 AC_LANG_POP([C++])
 CXXFLAGS=$save_CXXFLAGS