Allow passing in Android paths

Using `[target.arch.openssl]` Cargo sections isn't going to work that well here since 
with Android this will have to be built by the top level package and there's no way to pass global rustc linker flags 
from a top level package without editing all the Cargo files of the deps.
Sounds useful. I don't think any version of Android has openssl on it, we can use this opportunity for some helpful error reporting.