title: Are CMAKE_SOURCE_DIR and PROJECT_SOURCE_DIR the same in CMake?
body: <p><a href="https://gitlab.kitware.com/cmake/community/wikis/doc/cmake/Useful-Variables" rel="noreferrer">This page</a> contains a good summary of variables CMake already defines for us. I feel that some variables are the same. Take the example of <code>CMAKE_SOURCE_DIR</code> and <code>PROJECT_SOURCE_DIR</code> for example. They are the same, referring to the folder where the top level CMakeLists.txt is defined. So my question is: are there subtle difference between them? Thanks.  </p>