CMake >= 3.0 supports bracket comments, and the reStructuredText
integration code in sphinx/ext/ecm.py already supports extracting
the docs from a bracket comment instead.
Editing documentation without leading line comment markers is more simple,
e,g. when reflowing text over lines.

With ECM meanwhile requiring CMake 3.5 now it is possible to switch
(and thus follow also the approach used by cmake itself).

NO_CHANGELOG

The --sort=name option was only added in version 1.28
which isn't available in Centos 7 yet e.g.

- Search for a suitable `tar` program
- Check version output if it's GNU tar

Since this function is typically run only once per repo,
there's not much point in doing extra cache work or writing
a separate find-module.

With this MR cmake gets rerun when the dirs/subdirs of the template change.

And by setting the `DEPENDS` property of the tar command to this list of the dirs/subdirs of the template we ensure that the command gets executed whenever the files change.

By default tar glues files in a random order together, this makes the
output non reproducible. In order to fix, produce a sorted output and
uniquify user/group and chmod.