Strict dependency on fnmatch was removed in 52076d19.

We still have an optional dependency on it from CTags, but I don't
think we use this code, and anyways the checks we had for the optional
path to be taken were incorrect, so it was never actually used.

So, drop the dependency on libibrety for Windows builds, and fix the
remaining checks for an optional fnmatch so if it is found it can
actually be used.