The files in this directory originally come from
https://github.com/percona/PerconaFT/.

This directory only includes the "locktree" part of PerconaFT, and its
dependencies.

The following modifications were made:
- Make locktree usable outside of PerconaFT library
- Add shared read-only lock support

The files named *_subst.* are substitutes of the PerconaFT's files, they
contain replacements of PerconaFT's functionality.