Basic build infrastructure for Python scripts

Basic build infrastructure for Python scripts

This patch adds basic boilerplate support (based on corresponding Perl
sections) for enabling the building and installation Python scripts.

There are currently no Python scripts being built, and when Python
scripts are added in future patches, their building and installation
can be disabled by defining NO_PYTHON.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>