allow for package manager defaults (brew on osx)

This would allow us to set library locations based on parameters set by package managers, etc.  Right now they seem to be hard coded.

E.g., the specification for homebrew on osx would be:

```make os=osx package_manager=brew```

You may wish to poll ``` brew ``` for its prefix rather than set it automatically to ``` /usr/local ```. Despite Homebrew's urging, Homebrew is often installed outside of ``` /usr/local ```. 

You can get brew to give you its prefix with ``` brew --prefix ```, and then integrate that dynamic response in the same way you would handle shell commands normally in Makefiles.