x11-themes/gtk-arc-themes: Speed up build

The build is insanely slow.  Inkscape tries to connect to 127.0.0.1:9090
(probably gtk3's broadway backend) which has to timeout first until
the next inkscape call.  If this would only happen a couple of times
it would be ok but it happens >500 times.  Disable all GDK backends
to prevent that.

Unable to init server: Could not connect to 127.0.0.1: Connection refused

http://package18.nyi.freebsd.org/data/122amd64-default-foo/2021-04-05_21h03m50s/logs/errors/gtk-arc-themes-20210127.log

x11-themes/gtk-arc-themes/Makefile

@@ -31,5 +31,9 @@ MESON_ARGS=	-Dthemes=cinnamon,gnome-shell,gtk2,gtk3,metacity,plank,unity,xfwm \
 		-Dcinnamon_version=4.8 \
 		-Dgnome_shell_version=3.38 \
     		-Dgtk3_version=3.24
+# Speed up build in Poudriere.  It calls inkscape a bazillion times
+# and each instance tries to connect to 127.0.0.1:9090 which seems
+# to be because of gtk3's broadway backend.  Disable all GDK backends.
+MAKE_ENV=	GDK_BACKEND=""
 
 .include <bsd.port.mk>