+ ])
+
+# Pass -Werror as an extra flag during the test: this is needed to make the
+# -Wunknown-warning-option diagnostic fatal with clang.
+AC_LANG([C++])
+AX_APPEND_COMPILE_FLAGS([WARN_FLAGS_LIST], [WARN_CXXFLAGS], [-Werror])
+AC_LANG([C])
+AX_APPEND_COMPILE_FLAGS([WARN_FLAGS_LIST], [WARN_CFLAGS], [-Werror])