title: Get variables and options from external project
body: <p>I'd like to know how to get variables and options from an external project. For example, I'm using <code>ExternalProject_Add</code> to add a library that my project depends on, the thing is that this dependency has some variables and options - some of them are on by default - I'm wondering, how to get them from my <code>CMakeLists.txt</code>.</p>