doc: Enable book index in XSLT builds

The XSLT toolchain requires an empty <index> element where the index is
supposed to appear.  Add that with conditionals to hide it from the
DSSSL build.

doc/src/sgml/Makefile

@@ -251,7 +251,7 @@ regress_README.html: regress.sgml
 # if we try to do "make all" in a VPATH build without the explicit
 # $(srcdir) on the postgres.sgml dependency in this rule.  gmake bug?
 postgres.xml: $(srcdir)/postgres.sgml $(ALMOSTALLSGML)
-	$(OSX) -D. -x lower $< >postgres.xmltmp
+	$(OSX) -D. -x lower -i include-xslt-index $< >postgres.xmltmp
 	$(PERL) -p -e 's/\[(aacute|acirc|aelig|agrave|amp|aring|atilde|auml|bull|copy|eacute|egrave|gt|iacute|lt|mdash|nbsp|ntilde|oacute|ocirc|oslash|ouml|pi|quot|scaron|uuml) *\]/\&\1;/gi;' \
 	           -e '$$_ .= qq{<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">\n} if $$. == 1;' \
 	  <postgres.xmltmp > $@

doc/src/sgml/filelist.sgml

@@ -196,3 +196,8 @@
  to include it.
  -->
 <!ENTITY % include-index "IGNORE">
+
+<!--
+ Create empty index element for processing by XSLT stylesheet.
+ -->
+<!ENTITY % include-xslt-index "IGNORE">

doc/src/sgml/postgres.sgml

@@ -270,5 +270,6 @@
 
  &biblio;
  <![%include-index;[&bookindex;]]>
+ <![%include-xslt-index;[<index></index>]]>
 
 </book>