Only require METIS if ALUGrid is built for parallel

If we have a parallel ALUGrid, then we must also link with METIS. If
METIS is not available, then parallel ALUGrid should be disabled. If
we have a serial-only ALUGrid, then METIS is not required.
Merge pull request #324 from rolk/324_alugrid

Only require METIS if ALUGrid is built for parallel