title: Changing CMake build directory after running, within CMakeLists.txt
body: <p>Is there anyway of changing the CMake build directory from CMakeLists.txt file?
  I was looking at the CMAKE_BINARY_DIR variable but it seems to be unchangeable.
  The reason is that my project can be compiled for different platforms and I want for the user to define one build directory and the CMake will decide on the build location according to the platform:
  if (x86): CMAKE_BINARY_DIR = CMAKE_BINARY_DIR/x86
  if (x64): CMAKE_BINARY_DIR = CMAKE_BINARY_DIR/x64</p>
  
  <p>In short, to dismiss the user from configuring a build directory for each of the platform he wants to compile the project for.</p>
  
  <p>Thanks,
  Aner</p>