databases/rrdtool: Make groff a conditional dependency

groff is required by rrdtool to format documentation, which
is only required when documentation is being built.

Accordingly, make groff conditional on the existing DOCS option.

This precludes building a litany of dependencies, mostly due
to groff requiring ghostscript.

While here, remove unneeded post-patch in the case the DOCS option is
switched off.

Submitted by:	koobs
Differential Revision:	https://reviews.freebsd.org/D14732

databases/rrdtool/Makefile

@@ -3,7 +3,7 @@
 
 PORTNAME=	rrdtool
 PORTVERSION=	1.7.0
-PORTREVISION=	1
+PORTREVISION=	2
 CATEGORIES=	databases graphics
 MASTER_SITES=	http://oss.oetiker.ch/rrdtool/pub/
 
@@ -20,7 +20,7 @@ CPPFLAGS+=	-I${LOCALBASE}/include -I${LOCALBASE}/include/freetype2
 USE_RC_SUBR=	rrdcached
 USE_LDCONFIG=	yes
 GNU_CONFIGURE=	yes
-USES=		gmake groff libtool pathfix pkgconfig shebangfix
+USES=		gmake libtool pathfix pkgconfig shebangfix
 USE_GNOME=	glib20 libxml2
 INSTALL_TARGET=	install-strip
 
@@ -38,6 +38,10 @@ PYTHON_DESC=	Build PYTHON bindings
 RUBY_DESC=	Build RUBY bindings
 
 DEJAVU_RUN_DEPENDS=	dejavu>0:x11-fonts/dejavu
+
+DOCS_CONFIGURE_ENABLE=	docs
+DOCS_USES=		groff
+
 GRAPH_CONFIGURE_OFF=	--disable-rrdcgi --disable-rrd_graph
 GRAPH_LIB_DEPENDS=	libfreetype.so:print/freetype2 \
 			libpng.so:graphics/png
@@ -76,10 +80,6 @@ post-extract:
 	@${REINPLACE_CMD} -E 's|(^rrdtool_LDADD = librrd.la)|\1 $$\(ALL_LIBS\)|' \
 		${WRKSRC}/src/Makefile.in
 
-post-extract-DOCS-off:
-	@${REINPLACE_CMD} -e '/^SUBDIRS = /s| doc | |' \
-		${WRKSRC}/Makefile.in
-
 post-extract-EXAMPLES-off:
 	@${REINPLACE_CMD} -e '/^SUBDIRS = /s| examples | |' \
 		${WRKSRC}/Makefile.in