Log Message:
Import libxshmfence-1.1 as wip/libxshmfence.

This library offers a CPU-based synchronization primitive compatible
with the X SyncFence objects that can be shared between processes
using file descriptor passing.

There are two underlying implementations:

 1) On Linux, the library uses futexes

 2) On other systems, the library uses posix mutexes and condition
    variables.

TODO:
On NetBSD, two functions are missing:
pthread_mutexattr_setpshared and pthread_condattr_setpshared.
-- wiz 20131129