Restore --enable-ivsalloc.

However, unlike before it was removed do not force --enable-ivsalloc
when Darwin zone allocator integration is enabled, since the zone
allocator code uses ivsalloc() regardless of whether
malloc_usable_size() and sallocx() do.

This resolves #211.

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -186,6 +186,12 @@
 #undef JEMALLOC_INTERNAL_FFSL
 #undef JEMALLOC_INTERNAL_FFS
 
+/*
+ * JEMALLOC_IVSALLOC enables ivsalloc(), which verifies that pointers reside
+ * within jemalloc-owned chunks before dereferencing them.
+ */
+#undef JEMALLOC_IVSALLOC
+
 /*
  * Darwin (OS X) uses zones to work around Mach-O symbol override shortcomings.
  */