Clang 17 compatibility

[vkrause]

See individual commits for details, mainly fixing new warnings that break the build due to -Werror being set here.

That however only fixes half of the overall problem, date still needs a solution. That is unlikely to get upstream fixes for C++20 compatibility it seems (as it itself has become part of C++20 in slightly modified form), but with that new API appearing in libc++ we end up with a few clashes and/or ambiguities.

[felixguendling]

Thank you!

Regarding the date library, the only change I did was to embed the IANA timezone database into the binary using our res library. This is useful to not depend on a installed timezone database or having to link cURL and rely on a working internet connection. I didn't check how the standard implementation accesses the timezone database on different systems. In my opinion, it would make sense to continue to embed the database into the binary.

[vkrause]

I don't think those changes are the problem here, it's mainly clashing ostream operator<< overloads it seems, upstream also has an open issues about that (HowardHinnant/date#773).

[felixguendling]

I was thinking about how to upgrade to the std:: version of the date library. However, looking at the compiler support it seems that the support is still "partial" which is not something I really want to experiment with - especially not if we can just stick to the date library which is really solid. Since we have our own fork of the date library anyway, we might as well fix the problems there.

A similar problem exists with the fmt library: moving to the std:: version is not really worth it. The fmt library also gets performance improvements and new features here and there. So the std:: version doesn't seem to be superior.

[vkrause]

motis-project/date#2 addresses the date problem.

[oviano]

It's definitely a challenge trying to use this excellent library with C++20 when targeting various platforms:

Android - doesn't have zones, formatting or writing of durations and time points to streams
iOS/macOS - no zones, but does have formatting and streaming
Linux - has streaming
Windows - has everything

Obviously the above is specific to the versions of compilers I am using, but generally the most recent official.

[felixguendling]

I guess I lack some context.

Regrading targets: currently we support MacOS (AppleClang), Windows (MSVC) and Linux (Clang 17 + GCC 13 for amd64 + aarch64) which is also tested via CI both in MOTIS as well as here. More targets (such as Android / iOS) are definitely possible (probably with slight changes here and there) but currently there's no use case so nobody invested the time.