(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.
