fix BOOST_ROOT condition.
fix BOOST_ROOT condition.


Former-commit-id: 3169e027a9ea52a133af44c7b2ebd4b7f7180cc2
xunzhang/dynet,xunzhang/dynet,chunyang-wen/dynet,danielhers/dynet,xunzhang/dynet,shuheik/dynet,chunyang-wen/dynet,chunyang-wen/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,xunzhang/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,clab/cnn,shuheik/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet,clab/cnn,clab/dynet,clab/dynet,chunyang-wen/dynet,clab/cnn,danielhers/dynet,clab/dynet,clab/dynet