Hi Robert:

Can you try this patch. This appears to be a known issue
with setuptools. It's not a great patch but it seems
to work when running setup directly and running 
pip install --target....

For those following at home, I deduce the prefix by
looking at sys.argv and parsing for --prefix. I recognize
--prefix either as a standalone arg in which case the
prefix directory is the next arg in the argv. Or I recognize
--prefix as a prefix and use the value after stripping the
first 9 characters (--prefix=).

Than I iterate over the data array where the data files are
and make them absolute using the prfix directory.