`port select python` and qtwebengine

[René J.V. Bertin]

Hi,

I just learned that the QtWebEngine build process has a hard dependency on python 2 and cannot be configured to use a specific python interpreter exclusively. In other words, it fails if a user apparently used `port select python` to make a python 3 interpreter the default one.

I think "base" doesn't have any mechanisms in place to avoid such failures? For instance, a `port select` implementation where the generic command (here, `python`) is not a symlink but a script that can be controlled via an env. variable or a dedicated cli argument (a bit like Qt's qtchooser; could address safety concerns)? Even a simple API to check within the Portfile if the user made an incompatible selection (and raise an error) would be preferable over letting the build fail with unknown side-effects.

R.