Allow the use of readlinkat over readlink.

This can be useful in situations where readlink is disallowed.

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -245,6 +245,12 @@
  */
 #undef JEMALLOC_LOG
 
+/*
+ * If defined, use readlinkat() (instead of readlink()) to follow
+ * /etc/malloc_conf.
+ */
+#undef JEMALLOC_READLINKAT
+
 /*
  * Darwin (OS X) uses zones to work around Mach-O symbol override shortcomings.
  */