OpenFst: 1.3.4 (updated)

OpenFst formula has been updated to the current version. Unlike the
previous formula, it now also passes two flags to `configure` which most
users will want (and which are needed for things which depend on
OpenFst). Once/if this pull request is accepted, I will make add two
other formulae for OpenGrm-Ngram and OpenGrm-Thrax, both useful tools
depending on OpenFst.