Distutilsflags

See the commit for mpi4py change to packages.yml, should be self-explanatory.

The reason I consider this a stopgap solution is that one can now not have a different set of flags per arch. A proper package configuration framework allows making choices about which parameters to pass to recipes based on the $arch and other information, but I want to put off as much as possible on that until I can deal with it in its full breadth. Please tell me if this is too limiting even for the coming month.