[date] Add official CMake targets support

[qis]

Howard Hinnant added an official CMake script with target exports. Breaking changes:

• The minimum C++ version is set to C++17.
• Can't build database support on Linux without enabling remote API.
• The unofficial:: prefix goes away, but a wrapper script is installed by this port.

I suggest removing the wrapper script after a transition period.

[qis]

What's the best way to fix this?

https://github.com/HowardHinnant/date/blob/c56f915/include/date/date.h#L139

I'm surprised it's not disabled earlier in the header file.

https://github.com/HowardHinnant/date/blob/c56f915/include/date/date.h#L102

I could add a target_compile_definitions(date PUBLIC HAS_UNCAUGHT_EXCEPTIONS=1) or set it to PRIVATE and let the user deal with it.

Setting /Zc:__cplusplus would be a clean fix that we apply in a custom toolchain file, but that requires a relatively recent compiler and can't be exported since it is likely to break user code.

Edit: Talked to Howard. Will submit a PR to the date repo and update this PR if it gets accepted.

[qis]

@dan-shaw We have fixed the C++ version detection in date.h.

The UWP build problem still persists since the library uses C++17 by default (which I find a good idea), but expects the compiler to still support std::unhandled_exception() since it's deprecated, not removed.

The library defines _SILENCE_CXX17_UNCAUGHT_EXCEPTION_DEPRECATION_WARNING on line 102 (checked to be sure), but I still get the following error:

date.h(1054,1): error C4996:  'std::uncaught_exception': warning STL4006: std::uncaught_exception() is deprecated in C++17. It is superseded by std::uncaught_exceptions(), plural. You can define _SILENCE_CXX17_UNCAUGHT_EXCEPTION_DEPRECATION_WARNING or _SILENCE_ALL_CXX17_DEPRECATION_WARNINGS to acknowledge that you have received this warning.

Strangely, this only happens when compiling for UWP targets.

What would be the appropriate fix for that? I can think of these:

1. Patch date.h to use std::unhandled_exceptions() in C++17 mode.
2. Patch date.h and add #pragma warning(disable : 4996) (personal favorite).
3. Patch CMakeLists.txt to disable warning with /wd4996 (bad idea).
4. Patch CMakeLists.txt to set the default C++ version to 14 (sounds reasonable).

Regarding option 4: I really like that you can overwrite this value in a custom toolchain file. The tz.h interface depends on it. Please don't force me to set it in portfile.cmake and break this functionality.

Is this a known MSVC bug or did I miss something?

Edit: I force-pushed option 2.

[dan-shaw]

LGTM, the uwp patch looks fine - just one comment

[dan-shaw]

Thanks for the PR!