Makefile: extract script to massage Python scripts

Makefile: extract script to massage Python scripts

Extract a script that massages Python scripts. This provides a couple of
benefits:

  - The build logic is deduplicated across Make, CMake and Meson.

  - CMake learns to rewrite scripts as-needed at build time instead of
    only writing them at configure time.

Furthermore, we will use this script when introducing Meson to
deduplicate the logic across build systems.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>