Enable overriding JEMALLOC_{ALLOC,FREE}_JUNK.

This resolves jemalloc#509.

Author: jasone

include/jemalloc/internal/util.h

@@ -41,8 +41,12 @@
 #define	MALLOC_PRINTF_BUFSIZE	4096
 
 /* Junk fill patterns. */
-#define	JEMALLOC_ALLOC_JUNK	((uint8_t)0xa5)
-#define	JEMALLOC_FREE_JUNK	((uint8_t)0x5a)
+#ifndef JEMALLOC_ALLOC_JUNK
+#  define JEMALLOC_ALLOC_JUNK	((uint8_t)0xa5)
+#endif
+#ifndef JEMALLOC_FREE_JUNK
+#  define JEMALLOC_FREE_JUNK	((uint8_t)0x5a)
+#endif
 
 /*
  * Wrap a cpp argument that contains commas such that it isn't broken up into