look up python-config using AC_PATH_TOOL

Merged Helmut Grohne requested to merge helmutg/apparmor:python-cross-compilation into master

Doing so adds the $ac_tool_prefix during cross compilation and will end up using the correct, architecture-dependent python-config.

This is the second and last upstreamable change from https://bugs.debian.org/984582. It looks a little simpler here, because apparmor evolved upstream compared to the Debian version. Fortunately, it got a lot simpler in the process.