-# Enable it here unconditionally. It is supported by GCC >= 4.8 and by Clang
-# (it is accepted for compatibility although it has no effect), and there is
-# not reason to not want it.
-
-WARN_CFLAGS="${WARN_CFLAGS} -Wold-style-definition"
+# Enable them here unconditionally. They are supported by GCC >= 4.8 and by
+# Clang >= 4.0.
+WARN_CFLAGS="${WARN_CFLAGS} -Wold-style-definition -Wstrict-prototypes"