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.h.in

@@ -119,6 +119,13 @@ static const bool config_xmalloc =
     false
 #endif
     ;
+static const bool config_ivsalloc =
+#ifdef JEMALLOC_IVSALLOC
+    true
+#else
+    false
+#endif
+    ;
 
 #ifdef JEMALLOC_C11ATOMICS
 #include <stdatomic.h>