[gtkada] Patch: Add a command variable for Python.

[Emmanuel Briot]

> Given that binding.py works only with Python 2.7, some users may need
> to specify a different command to get the right version of the
> interpreter. Python 2.6 might still be the default somewhere, and in a
> not too distant future it may become increasingly necessary to invoke
> the Python 2 interpreter as "python2", as the migration to Python 3
> progresses. This patch makes the command customizable.

We do not, in general, expect users to use binding.py directly (and certainly
not as part of an automatic install on a distribution, better to use the provided
pre-generated source files).

regards
Emmanuel