Use dirsync for doxygen directory syncing

[carlescufi]

This commit introduced a mechanism to preserve mtime in Doxygen-generated XML files in order to avoid having to process unchanged files every time we incrementally build the doc. More info can be found in this thread.

The dirsync library seems up to the task, so potentially it could be used to replace much of the code in the script.

[marc-hb]

Thanks @carlescufi for filing this. I'm the one to blame for the additional 100 lines of restore_modification_times.py and I agree it would be nice to get rid of most of them and replace them by a rsync-like python library as very well explained in the doxygen mailing-list thread above.
Summarizing a private discussion with @carlescufi : the most critical requirement is obviously the ability NOT to copy/update the modification timestamp when it is newer but the content has not changed. This is more or less the --checksum option of rsync.
Also important is the equivalent of rsync's --delete option to remove renamed or deleted XML files.

As usual, the library replacement needs to be evaluated with respect to quality, maintainer responsiveness etc.

[marc-hb]

This commit introduced...

Ouch, I spent ages last week looking for this old issue and gave up eventually. Mentioning a commit links only one way. Mentioning PR #13159 here links both ways and would have let me find this issue :-)

[gmarull]

doxyrunner replaced the mechanism described here, closing.