The official goal is to be able to put libprocess and stout into a call to `ExternalProject_Add`, rather than having them built in-tree as they are now. Since Libprocess and Stout depend on a few variables being defined by the project that is building against it (such as, e.g., the `LINUX` variable) this will involve, at minimum, figuring out which `-D` flags have to be passed through the `ExternalProject_Add` call.
NOTE: This goal may not be feasible. We will need to trigger a rebuild of many source files if we change a header in Libprocess or Stout, and a relink if we change a .cpp file in the source files of Libprocess. This might require a fair bit of effort.
Another complication is that `StoutConfigure` manages the dependencies of Stout, and Stout is built through `ExternalProject_Add`, we will need to make sure this is managed in roughly the same way it is now.