add no_cups flavor to print/ghostscript

This patch by RegisteredOnlyToSay[1] adds the no_cups flavor to
print/ghostscript.

> [...] If you look at ports/print/ghostscript/gnu/Makefile you'll see
> that all cups dependencies are already pretty much separated. Just
> edit it slightly and make package.
>
> I added no_cups flavor to my ghostscript Makefile. Here is very
> straightforward patch.
>
> Feel free to send this patch to openbsd-ports if you want.

[1]:
https://old.reddit.com/r/openbsd/comments/g7d2eu/why_does_ghostscript_have_avahi_as_dependence/foypuy5/

--- Makefile.~1.114.~ Fri Jul 12 23:48:55 2019
+++ Makefile Sun Apr 26 04:22:42 2020
@@ -16,7 +16,7 @@

MAINTAINER= Matthias Kilian <kili@openbsd.org>

-FLAVORS= a4 gtk no_x11
+FLAVORS= a4 gtk no_x11 no_cups
FLAVOR?=

FLAVOR_STRING= ${FLAVOR_EXT:S/-/,/g}
@@ -85,7 +85,10 @@

USE_GMAKE= Yes

-ALL_TARGET = so all ${ALL_TARGET_CUPS}
+ALL_TARGET = so all
+.if !${FLAVOR:Mno_cups}
+ALL_TARGET += ${ALL_TARGET_CUPS}
+.endif
INSTALL_TARGET = install install-so

SOC_LIBS = -liconv -pthread
@@ -289,6 +292,7 @@
# Aditional devices not mentioned in configure.ac:
DEVICES+= ijs

+.if !${FLAVOR:Mno_cups}
# Cups
LIB_DEPENDS+= print/cups,-libs
WANTLIB += cups cupsimage
@@ -304,5 +308,6 @@
CUPSPDFTORASTER=1 \
CUPS_FONTPATH="${LOCALBASE}/share/cups/fonts" \
CUPS_DATA="${LOCALBASE}/share/cups"
+.endif

.include <bsd.port.mk>