Add sub8x8_mc experimental flag

Change-Id: Ifcc329df240c0771172180933a6180b21fd31abe

configure

diff --git a/configure b/configure
index a95c681..3c59c6e 100755
--- a/configure
+++ b/configure
@@ -260,6 +260,7 @@
     dual_filter
     ext_tx
     tx64x64
+    sub8x8_mc
     ext_intra
     filter_intra
     ext_inter