configure.ac: make "xmlto" optional

Makefile.am

@@ -569,9 +569,6 @@ endif
 # Documentation
 #
 
-DOCBOOK_FILES = doc/protocol.xml
-DOCBOOK_HTML = $(patsubst %.xml,%/index.html,$(DOCBOOK_FILES))
-
 man_MANS = doc/mpd.1 doc/mpd.conf.5
 doc_DATA = AUTHORS COPYING NEWS README TODO UPGRADING doc/mpdconf.example
 
@@ -579,12 +576,24 @@ if ENABLE_DOCUMENTATION
 protocoldir = $(docdir)/protocol
 protocol_DATA = $(wildcard doc/protocol/*.html)
 
+if HAVE_XMLTO
+
+DOCBOOK_FILES = doc/protocol.xml
+DOCBOOK_HTML = $(patsubst %.xml,%/index.html,$(DOCBOOK_FILES))
+
 $(DOCBOOK_HTML): %/index.html: %.xml
 	$(XMLTO) -o $(@D) --stringparam chunker.output.encoding=utf-8 html $<
 
 doc/api/html/index.html: doc/doxygen.conf
 	$(DOXYGEN) $<
 
+else
+
+DOCBOOK_FILES =
+DOCBOOK_HTML =
+
+endif
+
 all-local: $(DOCBOOK_HTML) doc/api/html/index.html
 
 clean-local:

configure.ac

@@ -1089,11 +1089,8 @@ AC_ARG_ENABLE(documentation,
 
 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$DOXYGEN = x; then
@@ -1101,6 +1098,8 @@ if test x$enable_documentation = xyes; then
 	fi
 
 	AC_SUBST(DOXYGEN)
+else
+	AM_CONDITIONAL(HAVE_XMLTO, false)
 fi
 
 AM_CONDITIONAL(ENABLE_DOCUMENTATION, test x$enable_documentation = xyes)