SynoCommunity/spksrc — comment on commit 512698680ea9a9db3e625bd1204947c1a7225f2b
File: cross/python27/PLIST line 4
By Diaoul:

Cannot import setuptools without this. The error is like here: https://dev.openwrt.org/ticket/8032

I guess this is because setuptools will use the same compilation parameters as Python to compile packages and thus, requires an include file to store it in the first place.
A bit awkward to have an include file I agree, but without that can't import setuptools and so can't install python modules, even pure python ones.