doc: make DocBook validation optional

It turns out that changes in the building environment, the version of
Doxygen being a prime suspect, can break the validation. Invalid DocBook
XML does lead to likely broken documentation, but perhaps it is better
than failing to build or having to disable documentation completely.

CI turns DocBook validation on, because the CI environment is stable and
known, and we do want to catch mistakes in hand-written DocBook files.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

doc/publican/meson.build

@@ -94,11 +94,16 @@ foreach src : files([
 	)
 endforeach
 
+skip_validation = '--skip-validation'
+if get_option('docbook_validation')
+	skip_validation = []
+endif
+
 custom_target(
 	'Wayland-docbook-html',
 	command: [
 		xmlto,
-		'--skip-validation',
+		skip_validation,
 		'--stringparam', 'chunker.output.encoding=UTF-8',
 		'--stringparam', 'chunk.section.depth=0',
 		'--stringparam', 'toc.section.depth=1',