Turn on var-tx by default

Change-Id: I4756594a6975ad0fff1f4ff64f1cf02876757f02
diff --git a/configure b/configure
index a112a4c..2c30bb2 100755
--- a/configure
+++ b/configure
@@ -490,6 +490,7 @@
     soft_enable ext_delta_q
     soft_enable loopfiltering_across_tiles
     soft_enable ec_smallmul
+    soft_enable var_tx
 
     # Backwards/jenkins compatibility with --enable-aom-highbitdepth
     enabled aom_highbitdepth && enable_feature highbitdepth