graphics/graphviz: fix build with BSD make BSD make does not know the $< variable in explicit rules, it only knows it for inference rules like ".c.o". This resulted in $< expanding to an empty string, which in turn got the build stuck because it tried to read from stdin. But not in parallel mode, where stdin was redirected to an empty file, resulting in an empty PostScript file being installed. Fixes PR pkg/55539.

(rillig)

@@ -1,4 +1,4 @@
-# $NetBSD: Makefile,v 1.217 2020/08/03 22:38:22 tnn Exp $
+# $NetBSD: Makefile,v 1.218 2020/08/09 16:04:44 rillig Exp $
 
 DISTNAME=	graphviz-2.44.1
 CATEGORIES=	graphics
@@ -19,6 +19,9 @@
 USE_LANGUAGES=		c c++
 USE_LIBTOOL=		yes
 USE_TOOLS+=		automake bison flex groff pkg-config
+# GNU make is needed since BSD Make does not know $< in explicit rules,
+# such as used by lib/cdt/Makefile.am.
+USE_TOOLS+=		gmake
 GNU_CONFIGURE=		yes
 GNU_CONFIGURE_STRICT=	no # has sub-configures
 CONFIGURE_ARGS+=	--disable-java