title: CMake: How to build an external project using all cores on *nix systems?
body: <p>I'm working on a CMake based project that has a dependency on a gigantic third party codebase that also uses CMake. I'm including the third party project via the ExternalProject_Add mechanism. That project defaults to using Makefiles, although the parent is an Xcode project.</p>
  
  <p>The resulting build of the external project is <strong>painfully slow</strong> because it's only using a single core. I don't think that I can force the project to generate Xcode projects instead of Makefiles.</p>
  
  <p>Assuming that I'm stuck with Makefiles, how can I inform ExternalProject_Add to use all the cores available for the titanic third party project?</p>
  
  <p>Note that the addition of the inevitable '-j N' option (why doesn't 'make' do this by default?) needs to conditionally be present for the Mac and Linux builds, but not Windows/Visual Studio.</p>