configure.ac: make DocBook processing optional

Disable the HTML documentation generator by default.  Most users will
pick it from the web site.

configure.ac

@@ -1006,16 +1006,21 @@ dnl
 dnl Documentation
 dnl
 
-AC_MSG_CHECKING([for xmlto (DocBook processing)])
-AC_PATH_PROG(XMLTO, xmlto)
-if test x$XMLTO != x; then
+AC_ARG_ENABLE(documentation,
+	AS_HELP_STRING([--enable-documentation],
+		[build documentation (default: disable)]),,
+	[enable_documentation=no])
+
+if test x$enable_documentation = xyes; then
+	AC_PATH_PROG(XMLTO, xmlto)
+	if test x$XMLTO = x; then
+		AC_MSG_ERROR([xmlto not found])
+	fi
+
 	AC_SUBST(XMLTO)
-	AC_MSG_RESULT($XMLTO)
-else
-	AC_MSG_RESULT(no)
 fi
 
-AM_CONDITIONAL(HAVE_XMLTO, test x$XMLTO != x)
+AM_CONDITIONAL(ENABLE_DOCUMENTATION, test x$enable_documentation = xyes)
 
 
 dnl