title: Option to force either 32-bit or 64-bit build with cmake
body: <p>I would like to offer a way that always builds my target as a 32-bit or always as 64-bit executable executable with cmake independent of the host system (Adding the "-m32" or "-m64" flag for gcc, not sure yet what to do for other compilers).</p>
  
  <p>I can think of three ways to do this, which one should I use?</p>
  
  <ol>
  <li>an option (-DUSE32bit=true)</li>
  <li>a tool chain (-DCMAKE_TOOLCHAIN_FILE=64bit.toolchain)</li>
  <li>build types (-DCMAKE_BUILD_TYPE=release32)</li>
  </ol>
  
  <p>In my case the forced 32-bit build will be the default and should be easy to use. A forced 64-bit build is the also useful for some cases and should not be too difficult. Using the bit width of the host system rarely makes sense for my case and I don't want to support it.</p>
  
  <p>I found a related question here (<a href="https://stackoverflow.com/questions/5805874/the-proper-way-of-forcing-a-32-bit-compile-using-cmake">The proper way of forcing a 32-bit compile using CMake</a>) but the answers mostly discuss how it can be done at all, not how best to make it configurable.</p>