DEPENDENCY octave 4.0.0 too old for "isenv", introduced with Octave 9

[Andy1978]

with commit 38f9389
the package started using isenv but this was first introduced with Octave 9

(added with https://hg.octave.org/octave/rev/85255746fc21)

So the minimum version 4.0.0 in DESCRIPTION is too old.

Background: Question on discourse:
https://octave.discourse.group/t/serial-communication-using-serial-usb-cable-for-arduino-in-gnu-octave/6508/3

[lostbard]

I can change it to check for a empty string using getenv I guess - I would prefer to keep allowing older versions where possible (and where doesnt require too much additional work :) )