Elminate ENV VAR to configure build #223

I am not a big fan of using env-vars to configure the build, but it works.
In the future I would like to explore if we can use a macro to specify the guest runtime, we can specify native libraries to add as arguments to the macro, or the path to a precompiled runtime, etc.
But that would be a major refactor, and this is good enough.

Originally posted by @jprendes in #49 (review)