avoid conflicts caused by setting install.prefix in distutils.cfg

setting `install.prefix` in `distutils.cfg` causes
conflicts with other options set on the command-line,
such as `--user`. This adds a check to distutils command-line parsing
to unset `install.prefix` if such a conflict would be caused
by settings from a higher priority source (e.g. command-line).
python: update 2.7.8 bottle.
python3: update 3.4.1 bottle.