title: Cmake - select different c++ standard for different sources
body: <p>As the title suggest, I'd like to use cmake to build a project, and depending on the source file, enforcing a different c++ standard.</p>
  <p>The reason is : I am working on a library and would like to make it c++03 compliant for compatibility, but would like to use Google test suite which requires c++11.</p>
  <p>So the unit tests would be compiled with c++11, but I'd like my library to fail at compilation if there is reference to a c++11 only feature.</p>