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

[Gaetan Nadon]

On 15-12-10 03:08 PM, Andreas Boll wrote:
> @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.

I haven't touched this for years, but I noticed that backward
compatibility with older versions of docbook is provided by the
platform. I recall seeing some errors when a book required version 4.3, 
for example, could not be found because the latest installed version of
docbook was 4.1. On the other end, I think requiring an older version
(4.1 when 4.3 is installed) should cause no errors as platforms redirect
the older version to the newer version of docbook as the platform wants
to be  backwards compatible.

Note that only "point" versions are backward compatible, like 4.1 and
4.3, not 4.3 and 5.0. Version 5 is not backward compatible to version 4.
However a simple document should work regardless. To be sure, one can
test requesting a 4.3 document on a later platform where only version 5
is installed.

The alternate solution seems ok. It should be tested on both version 4
and 5. It skips all the compatibility issues which are very difficult to
sort out. That would save on service cost.

For the xorg macro itself to remain backward compatible, I would suggest
you keep the actual test, but if it fails, perform the new test you
suggest (either the well formed 4.3 docbook or the <x></x>). This way we
are certain not to introduce problems in older (very old) xorg tarballs
that are compiled with the newer version of the xorg-macro. That's why
the xorg macros must remain backward compatible for eternity.