Adding placeholders for new expts to be added

Change-Id: I38952cd55b91f35e5db45bc8e6a20ef25069c464
--ext-refs:   extended references - for multi-ref in nextgen
--ext-inter:  extended inter - for new_inter/copy_mode in nextgen
--ext-interp: for new interpolation
diff --git a/configure b/configure
index 8bb20b7..147a03b 100755
--- a/configure
+++ b/configure
@@ -268,6 +268,9 @@
     ext_tx
     misc_fixes
     ext_intra
+    ext_inter
+    ext_interp
+    ext_refs
 "
 CONFIG_LIST="
     dependency_tracking