Enable -Wundef, when supported.

This can catch bugs in which one header defines a numeric constant, and another
uses it without including the defining header. Undefined preprocessor symbols
expand to '0', so that this will compile fine, silently doing the math wrong.

include/jemalloc/internal/jemalloc_preamble.h.in

@@ -10,6 +10,7 @@
 
 #define JEMALLOC_NO_DEMANGLE
 #ifdef JEMALLOC_JET
+#  undef JEMALLOC_IS_MALLOC
 #  define JEMALLOC_N(n) jet_##n
 #  include "jemalloc/internal/public_namespace.h"
 #  define JEMALLOC_NO_RENAME