Provided MKL_ROOT as an env variable
Provided MKL_ROOT as an env variable

This change allows the user to simply use "cmake .. -DMKL=true" and provide MKL_ROOT as an environment variable, as already allowed for BOOST_ROOT.
Abasyoni/dynet,Abasyoni/dynet,Abasyoni/dynet,Peilun-Li/dynet,Peilun-Li/dynet,Peilun-Li/dynet