Use a stamp file for the XSLT HTML doc build

This way it works more like the DSSSL build, and dependencies are
tracked better by make.

Also copy the CSS stylesheet to the html directory.  This was forgotten
when the output directory was changed.

doc/src/sgml/Makefile

@@ -255,8 +255,12 @@ postgres.xml: $(srcdir)/postgres.sgml $(ALMOSTALLSGML)
 	rm postgres.xmltmp
 # ' hello Emacs
 
-xslthtml: stylesheet.xsl postgres.xml
+xslthtml: xslthtml-stamp
+
+xslthtml-stamp: stylesheet.xsl postgres.xml
 	$(XSLTPROC) $(XSLTPROCFLAGS) $(XSLTPROC_HTML_FLAGS) $^
+	cp $(srcdir)/stylesheet.css html/
+	touch $@
 
 htmlhelp: stylesheet-hh.xsl postgres.xml
 	$(XSLTPROC) $(XSLTPROCFLAGS) $^