Check for echo in PATH instead of hardcoding /bin/echo

[Bjørn Forsman]

Hi,

GNU Parallel depends on a real echo implementation and currently does
that by hardcoding /bin/echo. Instead of hardcoding the path, please
either look for echo at configure time, or run time. (If at runtime,
please add error handling / no silent failing.)

Background: I just discovered a regression in nixpkgs after it
upgraded parallel to version 20200822 and parallel insisted on
--max-line-length-allowed being only 324. It turned out to be because
of missing /bin/echo. If either of the above suggestions would be
implemented, that issue would not happen. (I submitted an immediate
fix for nixpkgs, but I think this should be handled upstream.)

Thanks.

Best regards,
Bjørn Forsman