Expose spdlog option to use std::format instead of internal fmt lib

[hdoc]

Now that std::format is available in recent compiler versions (Clang 17, GCC 13), it's possible to use the compiler-provided std::format instead of fmtlib.

spdlog already provides this functionality upstream and exposes it in CMake, but it's not an option with Meson.

This change adds support for that functionality to our wrap.

[neheb]

Might want to add a check if std::format can be used.

[hdoc]

@neheb added a check for the format header, let me know if that's what you had in mind.

[neheb]

required can take a feature option directly.

[hdoc]

I'm aware that required can take get_option('std_format')) directly but this whole branch is gated by the if get_option('std_format').enabled() anyway so the header check is always required.
If the option isn't specified in the first if statement, the has_header() check will never execute.
Setting required to true is a logical optimization.

This is what I think you're proposing, please clarify if my assumption is wrong:

if get_option('std_format').enabled()
  if meson.get_compiler('cpp').has_header('format', required: get_option('std_format'))
    spdlog_compile_args += '-DSPDLOG_USE_STD_FORMAT'
  endif
endif

[neheb]

The check is not enough actually.

If compiling in a C++ mode lower than 17, it will still succeed. I haven't tested if check_header works. That would be the cleanest solution.

In exiv2, I currently use

if not cpp.has_header_symbol('format', '__cpp_lib_format')
  deps += dependency('fmt')
endif

[hdoc]

Updated the check to use the new C++20 library feature test macros like you proposed, and verified it worked locally with GCC 13 on macOS.

One question: do we need to do any feature gating for the C++ standard version here? If a user is using C++17 and using the compiled_lib option I think this will fail. Perhaps we should hide the std_format option behind a is_in_cpp20_mode check like here. Please let me know what you think is best for the end-user.

[neheb]

I think it's best to modify the build file as such:

if meson.get_compiler('cpp').has_header_symbol('format', '__cpp_lib_format', required: get_option('std_format'))
  spdlog_compile_args += '-DSPDLOG_USE_STD_FORMAT'
else
  fmt_dep = dependency('fmt', version: '>=8.1', required: get_option('external_fmt').enabled())
  if fmt_dep.found()
    spdlog_dependencies += fmt_dep
    spdlog_compile_args += '-DSPDLOG_FMT_EXTERNAL'
  endif
endif

[hdoc]

Looks reasonable and passed tests locally. I didn't realize get_option() could be used in this way. Thank you

[neheb]

you don't need enabled() here.

[hdoc]

Fixed