Fix cmake bug giving relative Eigen path.
Fix cmake bug giving relative Eigen path.


Former-commit-id: 7974581047f2d27e588407a396e55b70b36a62eb
clab/cnn,clab/dynet,chunyang-wen/dynet,danielhers/dynet,shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,clab/cnn,shuheik/dynet,xunzhang/dynet,xunzhang/dynet,danielhers/dynet,clab/dynet,chunyang-wen/dynet,shuheik/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,clab/cnn,danielhers/dynet,xunzhang/dynet,clab/dynet,xunzhang/dynet