DistutilsOptionError: must supply either home or prefix/exec-prefix -- not both
Another solution* for Homebrew users is simply to use a virtualenv.
Of course, that may remove the need for the target directory anyway - but even if it doesn't, I've found --target works by default (as in, without creating/modifying a config file) when in a virtual environment.

*I say solution; perhaps it's just another motivation to meticulously use venvs...
I've been usually installed python packages through pip.
For Google App Engine, I need to install packages to another target directory.
I've tried:

pip install -I flask-restful --target ./lib

but it fails with:

must supply either home or prefix/exec-prefix -- not both

How can I get this to work?