With cmake, how would you disable in-source builds?

CMake has two undocumented options:
CMAKE_DISABLE_SOURCE_CHANGES and CMAKE_DISABLE_IN_SOURCE_BUILD

cmake_minimum_required (VERSION 2.8)

# add this options before PROJECT keyword
set(CMAKE_DISABLE_SOURCE_CHANGES ON)
set(CMAKE_DISABLE_IN_SOURCE_BUILD ON)

project (HELLO)

add_executable (hello hello.cxx)

–

andrew@manchester:~/src% cmake .
CMake Error at /usr/local/share/cmake-2.8/Modules/CMakeDetermineSystem.cmake:160 (FILE):
  file attempted to write a file: /home/andrew/src/CMakeFiles/CMakeOutput.log
  into a source directory.

/home/selivanov/cmake-2.8.8/Source/cmMakefile.cxx

bool cmMakefile::CanIWriteThisFile(const char* fileName)
{
  if ( !this->IsOn("CMAKE_DISABLE_SOURCE_CHANGES") )
    {
    return true;
    }
  // If we are doing an in-source build, than the test will always fail
  if ( cmSystemTools::SameFile(this->GetHomeDirectory(),
                               this->GetHomeOutputDirectory()) )
    {
    if ( this->IsOn("CMAKE_DISABLE_IN_SOURCE_BUILD") )
      {
      return false;
      }
    return true;
    }

  // Check if this is subdirectory of the source tree but not a
  // subdirectory of a build tree
  if ( cmSystemTools::IsSubDirectory(fileName,
      this->GetHomeDirectory()) &&
    !cmSystemTools::IsSubDirectory(fileName,
      this->GetHomeOutputDirectory()) )
    {
    return false;
    }
  return true;
}