Fix python3 shebang paths and add python option.

[jameshilliard]

Using absolute paths can result in the wrong python3 binary being used, such as when cross compiling using a non-system python3.

Use the normal python3 env shebang instead.

Also add a meson option to allow overriding the python path.

[hughsie]

I thought that was bad? https://jmmv.dev/2016/09/env-considered-harmful.html

[jameshilliard]

I thought that was bad? https://jmmv.dev/2016/09/env-considered-harmful.html

This seems to be referring to installed scripts that need a specific python interpreter with a known python path rather than build scripts like this, they are recommending one generate shebang lines for script installation and do not appear to be recommending using hard coded paths for build scripts. In general the behavior of using the python3 binary found in PATH is correct for build scripts AFAIU, hard coding like this does not work as it assumes /usr/bin/python3 is the correct python3 binary to use(which will not be the case in various circumstances).

[hughsie]

In fwupd (the main package) we use the python3 picked up by meson -- e.g. https://github.com/fwupd/fwupd/blob/main/generate-build/meson.build#L6C17-L6C24 and https://github.com/fwupd/fwupd/blob/main/meson.build#L306 -- would that work here?

[jameshilliard]

would that work here?

I think so, since directly executing the script with a specific python executable should ensure the incorrect shebang is ignored, although probably a good idea to fix the shebang either way.

[jameshilliard]

In fwupd (the main package) we use the python3 picked up by meson

Ported this option as well from fwupd.