Linker errors if MPICC is not set

[lukashuebner]

The linkMPI function does nothing, if MPICC is not set (and MPI should be searched among the installed libraries), resulting in linker errors for all MPI functions.

#Helper function for linking with MPI only if needed
function(linkMPI TOLINK)
    #We only want to try to find MPI outrselves if it wasn't provided in MPICC by user
    if("a$ENV{MPICC}" STREQUAL "a")
           #find_package(MPI REQUIRED)
           target_link_libraries(${TOLINK} MPI::MPI_C)
    endif()    
endfunction(linkMPI)