libstree 0.4.2 (new formula)

Adds [libstree](http://www.icir.org/christian//libstree/index.html), a generic suffix tree implementation written in C. 

This library provides a fast implementation for some interesting string operations, solving the [longest common substring](https://en.wikipedia.org/wiki/Longest_common_substring_problem) and [longest repeated substring](https://en.wikipedia.org/wiki/Longest_repeated_substring_problem) problems in linear time with the length of the string and is also used as an example in Aaron Patterson's [Writing Ruby C Extensions: Part 2](http://tenderlovemaking.com/2010/12/11/writing-ruby-c-extensions-part-2.html) and in lian's [ffi-stree gem](https://github.com/lian/ffi-stree).
I've contacted the maintainer and received an auto-reply saying they'll be unable to respond until at least October 6th. I'll update this PR as and when I hear from them.