This approach will not work with a "multiarch" enabled environment, and break cross builds on Debian and Ubuntu.

Afaics, the proposal assumes that the python executable for the target architecture is installed (which it is not for the multiarch cross-build layout), and that each target architecture has to be identified by it's own directory tree (again, not in the multiarch environment, you can install multiple targets into the same path).

I don't think that identifying the target by some path is the right way to go, and you should be able to identify the target by giving the target triplet as used by the configure parameters and then deduce the location from the target (or have an explicit location given).

The idea here is to use the platform identifier which we already had in the trunk before the PLATDIR was removed (the multiarch id or if not defined the platform). So by having a <target> specifier, you could deduce a path, or a <target>-python-config executable and you're done. No need to know about a path directly.  Of course python cross builds would have to install the <target>-python-config executable or symlink.

Minor issue: please s/xbuild/cross_build/.