Allow EC_MULTISYMBOL to be disabled with DAALA_EC.

It is now possible to use daala_ec without ec_multisymbol by running:
 ./configure --enable-experimental --disable-ec_mulitsymbol.

Change-Id: I55a0473b887846f01ef832059cdbe31aebebce05

configure

@@ -491,7 +491,7 @@ post_process_cmdline() {
     enabled new_tokenset && enable_feature ec_multisymbol
     enabled ec_multisymbol && ! enabled ans && soft_enable daala_ec
     enabled ec_multisymbol && ! enabled daala_ec && soft_enable ans
-    enabled daala_ec && enable_feature ec_multisymbol
+    enabled daala_ec && ! disabled ec_multisymbol && enable_feature ec_multisymbol
     if enabled rawbits && ! enabled daala_ec; then
       log_echo "rawbits requires daala_ec, so disabling rawbits"
       disable_feature rawbits