Log Message:
Import libxshmfence-1.2 from pkgsrc-wip as x11/libxshmfence.
(needed to build DRI3 support in MesaLib and modular-xorg-server on Linux)

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 three underlying implementations:

 1) On Linux, the library uses futexes

 2) On FreeBSD, the library uses umtx

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