configure.ac: make "xmlto" optional

Even when --enable-documentation is passed, don't abort if "xmlto" was
not found.

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)