m4/nut_check_python.m4: allow overriding of PYTHON{2,3}_SITE_PACKAGES

[ffontaine]

Allow the end-user to override PYTHON{2,3}_SITE_PACKAGES as this can be useful when cross-compiling

[jimklimov]

Thanks, seems reasonable :)

Let's see how CI likes it. Eventually, it would be great to also get a PR for updates to docs/config-prereqs.txt and/or docs/ci-farm-lxc-setup.txt about setting up a worker for cross-build scenarios in the NUT CI farm.

So far there are very occasional (manually triggered) runs in whole containers running under qemu for this or that platform - which is prohibitively slow to be done regularly, and sometimes blows up with unimplemented CPU instructions etc.

If there could be some mix, e.g. using a native (amd64) gcc or clang binary with appropriate resource file trees to produce ARM etc. binaries, and only use qemu-static to actually run them for make check check-NIT tests, that would probably be a workable balance. For best results, we
may also have to teach our AX_RUN_OR_LINK_IFELSE m4 macro about some caller-defined option that this cross-builder can also run the binaries it has just built (which is not the default case).