diff -aurN devel/tbb.orig/Makefile devel/tbb/Makefile
--- devel/tbb.orig/Makefile	2018-10-11 16:47:13.891718000 +0200
+++ devel/tbb/Makefile	2018-10-12 09:50:24.725432000 +0200
@@ -26,7 +26,8 @@
 		version="${PORTVERSION}"
 
 OPTIONS_DEFINE=	DOCS
-DOCS_BUILD_DEPENDS=	doxygen:devel/doxygen
+DOCS_BUILD_DEPENDS=	doxygen:devel/doxygen \
+		dot:graphics/graphviz
 
 USE_LDCONFIG=	yes
 ALL_TARGET=	default