Added Macports installation instructions.

Some libraries do no resolve for some reason, adding LDFLAGS option
solves this.

See [scipy 0.12 installation error (building from source)](http://ask.sagemath.org/question/2465/scipy-012-installation-error-building-from-source) and  [[SciPy-dev] undefined symbol errors in scipy svn](http://mail.scipy.org/pipermail/scipy-dev/2006-February/005277.html) for similar errors and solutions.