Allow EC_MULTISYMBOL to be disabled with DAALA_EC.
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

GrokImageCompression/aom,luctrudeau/aom,smarter/aom,GrokImageCompression/aom,luctrudeau/aom,luctrudeau/aom,luctrudeau/aom,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,luctrudeau/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,GrokImageCompression/aom