Restrict openfoam to using openmpi+thread_multiple (partially resolves #5428) (#5479)

- latest version requires openmpi+thread_multiple for collated output.

var/spack/repos/builtin/packages/openfoam-com/package.py

@@ -292,6 +292,10 @@ class OpenfoamCom(Package):
 
     provides('openfoam')
     depends_on('mpi')
+
+    # After 1712 require openmpi+thread_multiple for collated output
+    conflicts('^openmpi~thread_multiple', when='@1712:')
+
     depends_on('zlib')
     depends_on('fftw')
     depends_on('boost')