title: Conanfile settings when cross-compiling with CMake and toolchain file
body: <p>I read that the compiler should be specified when invoking Conan's build command, or in the <code>[settings]</code> section of a Profile.</p>
  
  <p>However, I am building using Cmake and already provide the compiler information using a <code>CMAKE_TOOLCHAIN_FILE</code>.  I found I can set this in the <code>[env]</code> section of the profile, using <code>CONAN_CMAKE_TOOLCHAIN_FILE</code>, but it is not considered in <code>settings</code>.</p>
  
  <p>Now I am confused about the potential of specifying a compiler which does not match that specified in the <code>CONAN_CMAKE_TOOLCHAIN_FILE</code>.</p>
  
  <p>What is the correct way to specify the compiler in this case?  </p>
  
  <p>Is it safe to replace the standard settings section with <code>CONAN_CMAKE_TOOLCHAIN_FILE</code>?</p>
  
  <p>i.e. in conanfile.py:<br>
  <code>settings = "CONAN_CMAKE_TOOLCHAIN_FILE"</code>  </p>
  
  <p>(Also, why isn't source specified in <code>settings</code>?  Surely this is the most common change which affects the output binary?)</p>