Enable ext-refs as default

Change-Id: I53c844724794e13116152d0492bbcdd3ece12d17

configure

diff --git a/configure b/configure
index 742bdde..8384a33 100755
--- a/configure
+++ b/configure
@@ -487,6 +487,7 @@
     soft_enable new_tokenset
     soft_enable ext_intra
     soft_enable mv_compress
+    soft_enable ext_refs
 
     # Backwards/jenkins compatibility with --enable-aom-highbitdepth
     enabled aom_highbitdepth && enable_feature highbitdepth