We automatically convert that to use sys.executable now which is
always available on all platforms (because we're running with it).
On some platforms like NetBSD, `python` doesn't exist, and you must
use a specific python version. On most other distros, `python` is
Python 2, and we don't want to depend on that.
Closes https://github.com/mesonbuild/meson/issues/695
All these scripts were being used as `find_program()`, so we do not
lose any test coverage by doing this.
This means replacing @PLAINNAME@ and @BASENAME@ in the outputs. This is
the same feature as generator().
This is only allowed when there is only one input file for obvious
reasons + failing test for this.