(The branch adds a new configure option, --with-python-install-dir.  It
takes an absolute path as its argument as that is what the --install-lib
option for setup.py uses.  The code in rt45407 is the simplest way I
could think of to properly handle all of the following cases:

  1. --prefix not specified, --with-python-install-dir not specified
  2. --prefix specified, --with-python-install-dir not specified
  3. --prefix not specified, --with-python-install-dir specified
  4. --prefix specified, --with-python-install-dir specified

where "proper handling" means "assuring the installed Python utilities
are usable".)