Log Message:
gtk3: drop cups from SUGGESTED_OPTIONS

cups was added as default recently without broad discussion.  pkgsrc
has long declined to endorse/require a particular approach to printing
(because of the multitude of strong opinions about printing).
Defaulting to cups requires that everyone install it, and some object.
In addition, cups does not build on macos, resulting in all packages
depending on gtk3 being broken.  The cups option is not enabled by
default on other packages.  It seems that this option is not necessary
for programs using gtk3 to print on a cups-using system.

I'm reverting this as the freeze coordinator so that 2018Q2 has the
longstanding behavior.  Longer term, the question of whether cups
should be a default option bears discussion, including about what the
cosequences are for those who do and don't use cups, whether it should
be per-package or global, and about whether we want to deviate on
systems that can't build cups.  (Note that we are only talking about
the defaults and binary packages built using the defaults; it's long
been possible for anyone to enable the cups options globally for their
own builds.)