Add CI test: distcheck with MPI
Some header files are only included when MPI is active (e.g. in the generalized solver), thus the normal distcheck does not fail if they are missing in the corresponding automake variables and are not distributed.
Some header files are only included when MPI is active (e.g. in the generalized solver), thus the normal distcheck does not fail if they are missing in the corresponding automake variables and are not distributed.