use system bz2 and/or zlib

Builds upon #427

To test:

```
make clean
./setup.py build_ext --libraries z,bz2 --include-dirs lib #uses system zlib & bz2
make clean
./setup.py build_ext --libraries z --include-dirs lib,third-party/bzip2 #uses system zlib & our copy of bzip2
make clean
./setup.py build_ext --libraries bz2 --include-dirs lib,third-party/zlib #uses system bz2 & our copy of zlib
```