Enable python3 for ninja bootstrap on macOS 11+

[dhomeier]

Fixes #932 (tested on 12.1-13.1 – no further py3 conversion needed).

[nieder]

This made ninja build on 12.6. I tried ledger and libnss3.0e which both use ninja, and they failed similarly. Is the python path hardcoded in the binary?

Can probably be fixed likewise.

Is it worth doing something like this to avoid having to duplicate the final command?

  if [ -x /usr/bin/python ]; then
    PYTHON=/usr/bin/python
  else
    PYTHON=/usr/bin/python3
  fi
  $PYTHON configure.py --bootstrap

[dhomeier]

This made ninja build on 12.6. I tried ledger and libnss3.0e which both use ninja, and they failed similarly. Is the python path hardcoded in the binary?

I rather don't think so; in meson builds e.g. the Python interpreter that is configured by meson setup builddir is then defined in the configuration files inside builddir like build.ninja, compile_commands.json and retrieved from there when running ninja -C builddir. I assume similar is done in those packages, but I cannot try to build them currently since I don't even get to build emacsen-common on Ventura.

[nieder]

LGTM for now.
I wasn't able to find any simple proof that ninja hardcodes the python used into the binary for later runtime events. If it turns out to be the case, we can always split ninja into /usr/bin/python and /usr/bin/python3 .info files (or merge them to use a single %p/bin/python3.XX.

Edit: never mind about python not being hardcoded. It is in fact hardcoded.