hdf5: default +unsupported to on

This flag can end up being required if a dependency tree ends up needing
hdf5+cxx+parallel, but nothing turns it on. Since the core spack code
puts this together, it ends up that the end user needs to specify it
manually. Instead, just assume an unsupported configuration (since these
*are* useful setups) and if anyone wants to seek upstream support,
masking the flag can be done.

var/spack/repos/builtin/packages/hdf5/package.py

@@ -45,7 +45,7 @@ class Hdf5(Package):
 
     variant('cxx', default=True, description='Enable C++ support')
     variant('fortran', default=True, description='Enable Fortran support')
-    variant('unsupported', default=False, description='Enables unsupported configuration options')
+    variant('unsupported', default=True, description='Enables unsupported configuration options')
 
     variant('mpi', default=False, description='Enable MPI support')
     variant('szip', default=False, description='Enable szip support')