title: CMake: set path to MPI headers and binaries manually
body: <p>I am developing an MPI application which requires to be run with a specific implementation of MPI (let's call it MPIvA). On my workstation, another implementation of MPI (let's call it MPIvB) is installed. </p>
  
  <p>My application is built using CMake and the <code>find_library(MPI)</code> obviously points to MPIvB. It compiles and runs without hassle.</p>
  
  <p>I compiled MPIvA on my workstation. How can I make CMake use these headers and binaries?</p>