Fix of cmake xmldoc dependencies chain.

Fix of cmake xmldoc dependencies chain.

This patch fixes an issues that xml documentation is generated on all builds regardless
if related files has been changed or not.

The patch adds a global xmldoc make target to which all generated html or man files are
added as build dependency. Each dependency itself depends on related CMakeLists.txt and
the xml file generated from the related xml.in file.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=64058
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>