Add docbook system depend

sysdep/docbook.mk

+
+DEPEND_DEPTH:=		${DEPEND_DEPTH}+
+DOCBOOK_DEPEND_MK:=	${DOCBOOK_DEPEND_MK}+
+
+ifeq (+,$(DEPEND_DEPTH))
+DEPEND_PKG+=		docbook
+endif
+
+ifeq (+,$(DOCBOOK_DEPEND_MK)) # ---------------------------------------------
+
+PREFER.docbook?=	system
+DEPEND_USE+=		docbook
+DEPEND_ABI.docbook?=	docbook>=0.6
+
+SYSTEM_SEARCH.docbook=	\
+	bin/docbook2pdf
+
+SYSTEM_PKG.Ubuntu.docbook=docbook-utils
+SYSTEM_PKG.Debian.docbook=docbook-utils
+
+endif # DOCBOOK_DEPEND_MK ---------------------------------------------------
+
+DEPEND_DEPTH:=		${DEPEND_DEPTH:+=}