Exclude "test_parallel_linearsolver" from non-MPI builds

The "sources_hook" runs after "OpmFiles" in "OpmLibMain".
Therefore, we must search the "tests_SOURCES" to exclude particular
tests from the build.

This fixes a build problem in the non-MPI case.
Explicitly group conditions

For some reason I was unable to produce correct criteria without
inserting explicit parentheses.  I don't know why that is, but
better safe than sorry.
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.
Don't reference OwnerOverlap* unless MPI is available

The class OwnerOverlapCopyCommunication is not defined unless MPI is
avilable.  Therefore, we cannot reference the type unless we know
that MPI is available in the current translation unit.
Merge pull request #1 from bska/no-mpi

Fix build issues in !HAVE_MPI case