barrier: add hand-written barrier implementation

Grabbed from:
http://stackoverflow.com/questions/8115267/writing-a-spinning-thread-barrier-using-c11-atomics

common benchmarks makefile definitions

add "lock-free queue"

From:
http://www.codeproject.com/Articles/153898/Yet-another-implementation-of-a-lock-free-circular

License:
http://opensource.org/licenses/bsd-license.php

.gitignore