configure.ac: Add a --without-python option.

configure.ac: Add a --without-python option.

Being able to be explicit about Python support (in addition to the
default of auto-detecting it) and acting upon the result, specifying
it as an option gains us more control about both dependencies and
the resulting build.

Furthermore, relying purely on auto-detection can lead to problems for
source-based distros. E. g. systemd being built before *both* 32-bit &
64-bit ABIs are installed will lead to build failures as systemd's
build system will pick up either 32-/64-bit Python, conclude both are
available and fail if that's not the case.