Add compound segment experiment flag

Change-Id: Id80f76ea2598daa93e693c5efe29b782abe33957
diff --git a/configure b/configure
index fd1d931..5220b2c 100755
--- a/configure
+++ b/configure
@@ -266,6 +266,7 @@
     intra_interp
     filter_intra
     ext_inter
+    compound_segment
     ext_interp
     ext_refs
     global_motion