configure.ac: require xmlto for --enable-documentation

Makefile.am

@@ -2141,19 +2141,11 @@ user_DATA = $(wildcard doc/user/*.html)
 developerdir = $(docdir)/developer
 developer_DATA = $(wildcard doc/developer/*.html)
 
-if HAVE_XMLTO
-
 DOCBOOK_HTML = $(patsubst %.xml,%/index.html,$(DOCBOOK_FILES))
 
 $(DOCBOOK_HTML): %/index.html: %.xml
 	$(XMLTO) -o $(@D) --stringparam chunker.output.encoding=utf-8 html --stringparam use.id.as.filename=1 $<
 
-else
-
-DOCBOOK_HTML =
-
-endif
-
 doc/api/html/index.html: doc/doxygen.conf
 	@$(MKDIR_P) $(@D)
 	$(DOXYGEN) $<

configure.ac

@@ -1733,8 +1733,11 @@ dnl Documentation
 dnl ---------------------------------------------------------------------------
 if test x$enable_documentation = xyes; then
 	AC_PATH_PROG(XMLTO, xmlto)
+	if test x$XMLTO = x; then
+		AC_MSG_ERROR([xmlto not found])
+	fi
+
 	AC_SUBST(XMLTO)
-	AM_CONDITIONAL(HAVE_XMLTO, test x$XMLTO != x)
 
 	AC_PATH_PROG(DOXYGEN, doxygen)
 	if test x$DOXYGEN = x; then
@@ -1742,8 +1745,6 @@ if test x$enable_documentation = xyes; then
 	fi
 
 	AC_SUBST(DOXYGEN)
-else
-	AM_CONDITIONAL(HAVE_XMLTO, false)
 fi
 
 AM_CONDITIONAL(ENABLE_DOCUMENTATION, test x$enable_documentation = xyes)