rect_tx_ext: work with var_tx

Change-Id: Ie2c34490dc50cb242bcd701308e6b55243883b15
diff --git a/configure b/configure
index c8e1f3e..0e8d933 100755
--- a/configure
+++ b/configure
@@ -555,6 +555,7 @@
     enabled ext_comp_refs && enable_feature var_refs
     enabled ext_comp_refs && disable_feature one_sided_compound
     enabled altref2 && enable_feature ext_refs
+    enabled rect_tx_ext && enable_feature rect_tx
 
     if ! enabled delta_q && enabled ext_delta_q; then
       log_echo "ext_delta_q requires delta_q, so disabling ext_delta_q"