Convenience change to allow multiple build directories with different configurations without git wanting to add them.
Convenience change to allow multiple build directories with different configurations without git wanting to add them.


Former-commit-id: 9c14cc22e3dc1a1845b5cfba32b4ada0aab54ac3
clab/cnn,clab/dynet,shuheik/dynet,xunzhang/dynet,clab/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,clab/cnn,clab/dynet,shuheik/dynet,chunyang-wen/dynet,clab/cnn,clab/dynet,xunzhang/dynet,shuheik/dynet,shuheik/dynet,xunzhang/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,chunyang-wen/dynet,shuheik/dynet,clab/dynet,chunyang-wen/dynet,danielhers/dynet,xunzhang/dynet,chunyang-wen/dynet,shuheik/dynet,shuheik/dynet,clab/dynet,danielhers/dynet