new formula: opengrm-thrax

OpenGRM-Thrax provides support for compiling context-sensitive rewrite
rules into weighted finite-state automata using OpenFST.

The patch is a single (no-op) line that causes compiler problems in some
environments. It has been reported upstream.

The test uses Thrax to convert cardinal numbers from digits (like `170`)
to strings (like `one hundred seventy`).

If accepted this would complete the OpenFST/OpenGRM suite.