Add MPI dependency for dune-common

mpihelper.hh needs to know HAVE_MPI in the same way as the compiled
library in order to generate a consistent interface.
Only use MPI if explicitly enabled

Running with MPI on a regular workstation will probably not increase
performance, so this should be disabled as default.
Enable MPI support from DUNE with --enable-parallel
Bugfix change from macro to function

All variables we introduce which is not parameters are in fact global
variables that may be overwritten by recursive invocation!
Bugfix expand variable definition
Search in dunecontrol build directories for other modules
Bugfix don't search for module twice!
Merge pull request #202 from rolk/202_mpi

Allow MPI to be found and opm-core configured the same way as DUNE