title: Add an external source directory to a CMake build
body: <p>I'm using Google Mock for my project, and the instructions say it's better to build the library along with the project, because different compiler flags can introduce bugs. I don't want to add the gmock/ directory to my repository; I'd rather have the sources as an external dependency and plug it into my build process. Which brings me to my question: how can I instruct CMake to pull the external source directory into the build (i.e. build it in my project's build directory)? I found a <a href="https://stackoverflow.com/questions/7980784/cmake-add-sub-directory-which-is-not-sub-directory-on-real-directory">similar question</a> here, but the answers requires a rigid directory placement, and I'd rather have that configurable. Any other way to do it?</p>