Use full path for Eigen directory
Use full path for Eigen directory

Otherwise CMake < 3.6 fails to find it.


Former-commit-id: 780db05f29f6788bb584e549b0ac69c0177acd8d
shuheik/dynet,clab/dynet,clab/cnn,clab/cnn,clab/dynet,clab/dynet,xunzhang/dynet,shuheik/dynet,chunyang-wen/dynet,chunyang-wen/dynet,danielhers/dynet,shuheik/dynet,clab/dynet,danielhers/dynet,shuheik/dynet,xunzhang/dynet,clab/dynet,xunzhang/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,clab/dynet,xunzhang/dynet,chunyang-wen/dynet,shuheik/dynet,clab/cnn,danielhers/dynet