title: How to make CMake targeting multiple platforms in a single build
body: <p>I'd like to configure my CMake project in a way, so that a single build execution targets multiple platforms (in my case I'd like to build for Linux and Windows, x86_32 and x86_64 targets each). I have the cross compiler toolchains installed and working, and building for each individual target works.</p>
  
  <p>So the challenge is setting up CMake in a way, that the toolchain and the <code>CMAKE_SYSTEM_…</code> variables are set appropriately for each sub-build. How can I do that?</p>