docbook/Makefile: option to not use local catalog

- if nocatalog=yes, the xsltproc is started without XML_CATALOG_FILES
  and the default ones from system are used

docbook/Makefile

@@ -113,6 +113,12 @@ ifeq ($(validate), 0)
 	override xsltproc_flags := $(xsltproc_flags) --novalid
 endif
 
+ifeq ($(nocatalog),yes)
+XMLCATALOGX=
+else
+XMLCATALOGX="XML_CATALOG_FILES=$(catalog)"
+endif
+
 all_deps = Makefile $(docbook_dir)/Makefile $(docbook_dir)/entities.xml \
 		   $(dep_xsl) $(catalog) $(extra_deps)
 
@@ -133,12 +139,12 @@ txt text plaintext: $(txt_files)
 readme README: ../README
 
 ../README: $(readme_docs) $(readme_deps) $(readme_xsl) $(all_deps)
-	XML_CATALOG_FILES=$(catalog) $(xsltproc) $(xsltproc_flags) \
+	$(XMLCATALOGX) $(xsltproc) $(xsltproc_flags) \
 		--xinclude \
         $(readme_xsl) $< | $(lynx) $(lynx_flags) -stdin -dump > $@
 
 $(output_dir)/%.html: %.xml %.d $(single_html_xsl) $(all_deps)
-	XML_CATALOG_FILES=$(catalog) $(xsltproc) $(xsltproc_flags) \
+	$(XMLCATALOGX) $(xsltproc) $(xsltproc_flags) \
         --xinclude \
         --stringparam base.dir "$(output_dir)/" \
         --stringparam root.filename "$(basename $<)" \
@@ -148,7 +154,7 @@ $(output_dir)/%.html: %.xml %.d $(single_html_xsl) $(all_deps)
 
 
 $(output_dir)/%.txt: %.xml %.d $(txt_xsl) $(all_deps)
-	XML_CATALOG_FILES=$(catalog) $(xsltproc) $(xsltproc_flags) \
+	$(XMLCATALOGX) $(xsltproc) $(xsltproc_flags) \
 		--xinclude \
         $(txt_xsl) $< | $(lynx) $(lynx_flags) -stdin -dump > $@
 
@@ -164,7 +170,7 @@ $(output_dir)/%.txt: %.xml %.d $(txt_xsl) $(all_deps)
 
 .PHONY: check
 check: $(docs) $(html_docs) $(txt_docs) $(readme_docs)
-	XML_CATALOG_FILES=$(catalog) $(xmllint) $(xmllint_flags) $<
+	$(XMLCATALOGX) $(xmllint) $(xmllint_flags) $<
 
 .PHONY: clean
 clean: