message: build: enable -Werror=gnu
  Stop the build if a warning is emitted due to `-Wgnu` and
  `--enable-werror` has been used. As usual - this would help notice such
  a warning that is about to be introduced in new code.
  This is a followup to
  build: ensure we aren't using GNU extensions
diff: @@ -365,6 +365,7 @@ if test "x$enable_werror" = "xyes"; then
  if test "x$CXXFLAG_WERROR" = "x"; then
  AC_MSG_ERROR("enable-werror set but -Werror is not usable")
  fi
  + AX_CHECK_COMPILE_FLAG([-Werror=gnu],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=gnu"],,[[$CXXFLAG_WERROR]])
  AX_CHECK_COMPILE_FLAG([-Werror=vla],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=vla"],,[[$CXXFLAG_WERROR]])
  AX_CHECK_COMPILE_FLAG([-Werror=switch],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=switch"],,[[$CXXFLAG_WERROR]])
  AX_CHECK_COMPILE_FLAG([-Werror=thread-safety-analysis],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=thread-safety-analysis"],,[[$CXXFLAG_WERROR]])
  