Search for MPI support

The build system depends on knowing if MPI is available.  Explicitly
search for MPI to honour that requirement.  Don't rely on ISTL's
transitive searching for the same.

cmake/Modules/opm-core-prereqs.cmake

@@ -6,6 +6,7 @@ set (opm-core_CONFIG_VAR
 	HAVE_ERT
 	HAVE_SUITESPARSE_UMFPACK_H
 	HAVE_DUNE_ISTL
+	HAVE_MPI
 	)
 
 # dependencies
@@ -28,6 +29,8 @@ set (opm-core_DEPS
 	"TinyXML"
 	# Ensembles-based Reservoir Tools (ERT)
 	"ERT"
+	# Look for MPI support
+	"MPI"
 	# DUNE dependency
 	"dune-common"
 	"dune-istl"