Builds only possible with MPI enabled

[mkuchnik]

Building with -DENABLE_MPI=OFF still results in an MPI build. This is because config/project.cmake forces ENABLE_MPI to be TRUE. Fixing this still results in build errors, since MPI dependencies are included for the compile.

[rfbird]

Currently MPI is required, and disabling the option doesn't make too much semantic sense. It's carried over from the underlying build system (cinch). For now we will just mark that as an advanced option and require MPI libraries for linking as a minimum.

Closed by 7ff52b7