Now adds correct library variables for boost
Adding armadillo find or install
Adding compile definitions with armadillo

Should really check for blas and lapack first. But since the idea is to later have a blas and lapack
install, I'll leave this for later.

These compile definitions should cover the "patch" for armadillo in the original build system.
External step where cmake calls itself

This allows external project to catch themselves once they have installed themselves.
This stuff is put in a separate script file, cos we are going to use it over and over.