[IDEA] Just use `python` from the $PATH?

Why cannot we just use `python` from the `PATH`? Like in this example PR.

That's how I detect which Python to use in cmake for example --- I just check for `python`. That's how `GMP` detects `m4` (see #217) --- it just looks it up in PATH. 

I noticed the `$PYTHON` is used for `python-host`. Does `python-host` work at all however?