[PATCH RFC macros] Fix XORG_WITH_XMLTO to work with xmlto >= 0.0.27

[Andreas Boll]

Starting with xmlto version 0.0.27 the return code of
  xmlto --skip-validation txt conftest.xml
is non-zero if conftest.xml is an empty file.

As a consequence the macro XORG_WITH_XMLTO returns
  xmlto cannot generate text format, this format skipped
and therefore libraries like libxi, libxdmcp and others won't convert
docbook XML to text format.

This changed behavior was introduced with the following change in xmlto:
  xmlto.in: use correctly exit code from xsltproc
See also: https://fedorahosted.org/xmlto/changeset/77

Peter Pearse suggested to use a valid docbook XML file.

More details could be found at:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=613674

Suggested-by: Peter Pearse <peter.pearse at linaro.org>
Cc: Gaetan Nadon <memsize at videotron.ca>
Signed-off-by: Andreas Boll <andreas.boll.dev at gmail.com>
---
@Gaetan:
Do you still have a concern on the patch?

> The one thing that bugs me about the patch is that the version of
> docbook may eventually be
> no longer available and this macro must remain backward compatible for
> eternity.

An alternate solution suggested by Helmut Grohne would be to simply add
<x></x> into conftest.xml.

 xorg-macros.m4.in | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/xorg-macros.m4.in b/xorg-macros.m4.in
index 9b3c503..672560b 100644
--- a/xorg-macros.m4.in
+++ b/xorg-macros.m4.in
@@ -428,6 +428,10 @@ fi])
 # Test for the ability of xmlto to generate a text target
 have_xmlto_text=no
 cat > conftest.xml << "EOF"
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
+                   "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd">
+<book id="empty_book">
+</book>
 EOF
 AS_IF([test "$have_xmlto" = yes],
       [AS_IF([$XMLTO --skip-validation txt conftest.xml >/dev/null 2>&1],
-- 
2.1.4