This is happening on Debian Testing currently since `python-docutils` was migrated to Python3, only Python3 is installed as build dependency. But `scripts/gen-api-gtkdoc.py` uses `#!/usr/bin/env python` as Python interpreter which is not available when only installed Python3. Maybe this is just temporary on Debian Testing and once the Python3 transition is done, they add also a generic `python` binary in $PATH. But anyway, since autotools check for a Python installed already for us, let's use its result.