Enable overriding JEMALLOC_{ALLOC,FREE}_JUNK.
This resolves #509.
diff --git a/include/jemalloc/internal/util.h b/include/jemalloc/internal/util.h
index 119696b..d9f9741 100644
--- a/include/jemalloc/internal/util.h
+++ b/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
