Change to building externals from source


We just merged a change that moves the external project targets into the build folder. This makes a ‘clean build’ (how we refer to removing the contents of your build directory) actually perform a true clean build.

If you build from source, you may get some weird error messages about missing libs. If so, clear your build directory and re-run cmake and you should be all set. Once you’re building again, you can remove the old build-ext folder.