Hard-coded /usr/bin paths

[peti]

Please don't hard-code search paths like /usr/include/zookeeper in the Cabal file! On many machines, this path doesn't exist, and on some distributions (like NixOS) the build will fail because of that setting. It's much better to trust the user to specify --extra-include-dirs properly rather than providing incorrect defaults.

[jpittis]

For those of us on OSX, brew install zookeeper creates /usr/local/include/zookeeper.

We can then add the following to our stack.yml.

extra-include-dirs:
- /usr/local/include/zookeeper

and stack build should now work.