- Follow the same logic for finding boost-python as for Linux.
- Set DYLD_LIBRARY_PATH so that ctypes find_library can find
boost-python.
- Do not use '-mt' suffix when linking against boost-serialization
and boost-program_options on Darwin.
The `buildPython*` function computes name from `pname` and `version`.
This change removes `name` attribute from all expressions in
`pkgs/development/python-modules`.
While at it, some other minor changes were made as well, such as
replacing `fetchurl` calls with `fetchPypi`.
