Add experimental flag for line graph transforms

Change-Id: Iba98b07d2f19fd98be07d0c2c80c855661df778d
diff --git a/configure b/configure
index 0735dba..b66491e 100755
--- a/configure
+++ b/configure
@@ -319,6 +319,7 @@
     smooth_hv
     var_refs
     rect_intra_pred
+    lgt
     sbl_symbol
 "
 CONFIG_LIST="