python now configures (thanks grknight, floppym)

* python_get_library_path function called die because the requiired
  variables were not being created by python_setup.

* Added conditional code for calls to econf variables, pkg_setup and
  src_prepare.

* Removed PYTHON_SINGLE_TARGET and added PYTHON_DEPS and
  PYTHON_USEDEP.

* Sorted some dependencies alphabetically and with conditional deps at
  the end of the list.