Explicitly linked to libtre

I've been playing around with libcrm114 (specifically its python bindings) and, while it would build fine, the module wouldn't import correctly:

```
roger@cristopher:~/workspace/libcrm114/tests$ python simple_demo.py 
Traceback (most recent call last):
  File "simple_demo.py", line 2, in <module>
    import pycrm114 as p
ImportError: /home/roger/workspace/libcrm114/lib/libcrm114.so.1: undefined symbol: tre_regfree
```

For some reason the python interpreter can find the libcrm114.so, but can't find libtre.so (despite the fact that it is installed; the C tests run fine).

I found that if I modified the Makefile to explicitly link to libtre, the python tests run fine.  I've included the change I made in this pull request.