Add a new experiment of var_refs

This coding tool is targeted to check redundant references available
to each frame at the frame level, and then try to save bits to avoid
coding redundant reference information at the block level.

Change-Id: I320178a9fe0cd806960318ac59659808675e5176
diff --git a/configure b/configure
index c1d511f..562b1be 100755
--- a/configure
+++ b/configure
@@ -318,6 +318,7 @@
     aom_qm
     lowdelay_compound
     smooth_hv
+    var_refs
 "
 CONFIG_LIST="
     dependency_tracking