Enable ext-comp-refs by default.

Change-Id: I858893147ee3cb369001bc094d8f848476019047
diff --git a/configure b/configure
index cf0fa05..b2faff6 100755
--- a/configure
+++ b/configure
@@ -521,6 +521,7 @@
     soft_enable compound_segment
     soft_enable interintra
     soft_enable one_sided_compound
+    soft_enable ext_comp_refs
     soft_enable smooth_hv
     soft_enable parallel_deblocking
     soft_enable rect_intra_pred