Add some glibc-like #defines for libchrome.

libchrome uses __USE_XOPEN2K8 to decide whether futimens is
available. That's perhaps not the best idea, but there are other
cases where we defined the same feature macros as glibc to aid
portability.

Change-Id: Ie6e04cb181d88698d618e7dbd26cd347a6bf076c