diff --git a/doc/Makefile b/doc/Makefile
index 4fe1f0c..022f76d 100644
--- a/doc/Makefile
+++ b/doc/Makefile
@@ -3,8 +3,9 @@ OBJ			:= $(SRC:.adoc=.pdf) $(SRC:.adoc=.html) $(SRC:.adoc=.ps)
 EXTRA_OBJ	:= $(SRC:.adoc=.xhtml)
 EXTRA_OBJ	+= $(SRC:.adoc=.xml) $(SRC:.adoc=.fo)
 ASCIIDOC	:= asciidoctor
-XSLTPROC	:= xsltproc
 FOP			:= fop
+XSLTPROC	:= xsltproc
+XSLT_PARAMS	?=
 DOCBOOK_XSL := /usr/share/xml/docbook/stylesheet/docbook-xsl-ns
 DOCBOOK_FO  := $(DOCBOOK_XSL)/fo/docbook.xsl
 
@@ -24,7 +25,7 @@ clean:
 	$(ASCIIDOC) -b docbook5 -o $@ $<
 
 %.fo: %.xml
-	$(XSLTPROC) -o $@ $(DOCBOOK_FO) $<
+	$(XSLTPROC) $(XSLT_PARAMS) -o $@ $(DOCBOOK_FO) $<
 
 %.pdf: %.fo
 	$(FOP) -fo $< -pdf $@