Fix ZeroC library linking when using `brew install --HEAD omero`

Error from [OMERO-homebrew-stable#137](http://hudson.openmicroscopy.org.uk/view/2.%20Stable/job/OMERO-homebrew-stable/137/OSX=10.7.5/console)
```
++ bin/brew install omero --HEAD
==> Installing omero dependency: autoconf
...
==> Checking out branch dev_4_4
==> ./build.py -Dice.home=/usr/local build-default
Error: No such file or directory - /usr/local/Cellar/zeroc-ice33/HEAD/python
```