This check fixes a probably rare case, where we land in a repository
that contains this clang-format.sh hook but where the format file went
missing. In such cases, clang-format falls back to the system defaults,
which brings undesired defaults, e.g. enforced line breaks in comments
after 90 chars, which cannot easily be reverted by a reformat.

Since clang-format's --fallback-style=none option is not available for
git clang-format, this comment reimplements the logic.

- Remove deprecated version checks
- Use VERSION_GREATER_EQUAL

In case one has uninstalled the project, but has made local changes that
should be tested the test would fail. Instead check if the files exist
before adding them to the list of files that will be tested.

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

If we tell the compiler to link the runtime statically and then explicitly link
the dynamic runtime and prohibit the static we continue to produce dynamic
builds...

This matches most of the existing code in KDE; also note that Qt has the
same setting (also stating in a comment in their _clang-format that this
matching their existing code).

To prevent cmake errors with exiting usages it is checked if the target already exists.