Ubuntu 20.04 LTS has reached the end of standard support. There no
longer appear to be compelling reasons to maintain compatibility with
CMake 3.16.
The new suggested minimum, CMake 3.22, is shipped with Ubuntu 22.04 LTS,
which is supported until April 2027.
This change also introduces new CMake policies, from CMP0098 to CMP0128.
To use, invoke `cmake` with argument `--preset dev-mode`.
Solves one item in #1235.
One disadvantage over `./configure --enable-dev-mode` is that CMake
does not provide a way to "hide" presets from users. That is,
`cmake --list-presets` will list dev-mode, and it will also appear
in `cmake-gui`, even though it's not selectable there due to bug
https://gitlab.kitware.com/cmake/cmake/-/issues/23341. (So in our
case, that's probably rather a feature than a bug.)
We curently use version 3 presets which require CMake 3.21+.
Unfortunately, CMake versions before 3.19 may ignore the `--preset`
argument silently. So if the preset is not picked up, make sure you
have a recent enough CMake version.
More unfortunately, we can't even spell this warning out in
CMakePresets.json because CMake does not support officially support
comments in JSON, see
- https://gitlab.kitware.com/cmake/cmake/-/issues/21858
- https://gitlab.kitware.com/cmake/cmake/-/merge_requests/5853 .
We could use a hack hinted at in
https://gitlab.kitware.com/cmake/cmake/-/issues/21858#note_908543
but that's risky, because it could simply break for future versions,
and we probably want to use presets not only for dev mode.
