title: how do I cahin logical CMake Generators?
body: <p>Logically I want to do something like this in my cmake build (given in pseudo C++)</p>
  
  <p><code>exceptions_flag = 
      Compiler == Clang || Compiler == GCC ? "-fexceptions"
    : Compiler == MSVC ? "/U_HAS_EXCEPTIONS;/D_HAS_EXCEPTIONS=1;/EHsc"
  </code></p>
  
  <p>I'm not sure how to do this in one step with cmake generators the best I can think of is</p>
  
  <p><code>set(EXCEPTIONS_FLAG 
  $&lt;$&lt;CXX_COMPILER_ID:MSVC&gt;:"/U_HAS_EXCEPTIONS;/D_HAS_EXCEPTIONS=1;/EHsc"&gt;
  if(NOT ${EXCEPTIONS_FLAG})
    set(EXCEPTIONS_FLAG "-fexceptions")
  endif()
  </code></p>
  
  <p>Is there a better way to do this?</p>