todo

利用C

* Add meta-dependency package for optional dependencies, where at least one
dependency must be satisfied:

A-la:

Dependencies = ["petsc" or "umfpack", "python"]

meaning we want either 'petsc' or 'umfpack', but we need at least one of them

Proposed syntaxes

matrx = Alternatives('petsc','umfpack')
Dependencies = [matrix, 'numpy-1', 'ufc']

or 

matrix = Dependency("petsc", "umfpack", type="any")
 -> type can be 'any' or 'all', defaults to 'all'
Again, we put matrix in the Dependencies list

We also need to set HAVE_<PACKAGE> flags accordingly