title: CMake: specifying build toolchain
body: <p>Very new to CMake, and so far I'm finding it to be extremely helpful. I have a set of custom libraries that I would like to build for multiple platforms using cross-compilation. The toolchains are installed, and I can hand-create the <code>Makefile</code>s that I need to do so, but I would like to be able to make use of CMake.</p>
  
  <p>Is there a way to tell <code>cmake</code> which toolchain to use, either at the command line or in the <code>CMakeLists.txt</code> file?</p>