psutils dependency

[Bruno Haible]

Hi,

During configure on macOS, I see this warning:

configure: WARNING: ===========================================================
configure: WARNING: a2ps works much better with psutils.  Available at:
configure: WARNING:   https://github.com/rrthomas/psutils/
configure: WARNING: You *really* should install it *before* installing a2ps.
configure: WARNING: ===========================================================

This optional dependency should also be mentioned in the DEPENDENCIES file.
So that the person who installs the package know up-front about the
prerequisites.

Bruno