Search in python path for libs to prevent need to source thisroot.sh

I recently built an anaconda (https://store.continuum.io/cshop/anaconda/) distribution of root, along with packages for rootpy/root_numpy. I built root with --prefix and did a lot of work to get the RPATH right so I don't need to source thisroot.sh every time. Everything seems to work fine for my conda packages, but the area in the rootpy code that I patched relies on the LD_LIBRARY_PATH (or equivalent) to be set. My modification makes it check the python path too, which will include the root lib directory in the case that you have a --prefix build.