Fix build directory handling in python_distutils build system.

Fix build directory handling in python_distutils build system.

* Apparently, Distutils does out of source tree building by default.
* Default build directory is "$srcdir/build".
* --build-base command line option is ineffective (some even fail)
  unless it is passed to the "build" command. However, if build-base is set in
  the config file, all setup.py commands use it (build, install and clean).
  That's a big flaw in Distutils design but it has been like this for a long
  time. Therefore write a custom distutils cfg file in the build directory
  to make build-base work. The best choice for config file path is
  $HOME/.pydistutils.cfg (one of the paths Distutils reads) and setting $HOME
  to the build directory.

Signed-off-by: Modestas Vainius <modestas@vainius.eu>