Vim fails to build with enabled dynamic Ruby intepreter when compiler flag for C23 standard is set

[zdohnal]

Steps to reproduce

Vim fails to build if it is configured for dynamically loaded Ruby plugin and C23 compiler flag is set. Steps to reproduce:

$ CFLAGS="${CFLAGS} -std=c23" CXXFLAGS="${CXXFLAGS} -std=c23" ./configure --enable-gui=no --enable-rubyinterp=dynamic
$ make

Here are build errors - the issue is C23 takes function with () as function with no argument, but Vim Ruby plugin code uses this way to declare functions which have a callback function as argument, but different functions with different number of arguments are used as callbacks in those declared functions.

I have tried to fix the issue by #16512 by using variadic arguments for the callback arguments, but this breaks Vim build with dynamically linked Ruby interpreter, because now the function declarations were different than how they are declared in Ruby header.
Ruby header anyargs.h actually generates every function with different number of arguments, and hides all of them under generic public macro, which is called from programs using Ruby, and the correct function is used based on number of arguments - unfortunately I was not able to mimic this macro processing in Vim.

I have worked it around for now by setting -std=c17 for compilation, until someone can come up with better solution.

(The other thing came up to my mind is to #ifdef those functions with variadic arguments specially for dynamically loaded Ruby plugin - if you would prefer this solution, do let me know, I can prepare PR for it)

Expected behaviour

Vim builds with dynamic Ruby plugin under C23.

Version of Vim

Current master patchlevel - 9.1.1071

Environment

OS: Fedora 41
TERM: xterm-256color
Compiler: gcc-14.2.1-7.fc41.x86_64 (but anything with C23 does the trick)
Ruby: ruby-devel-3.3.6-17.fc41.x86_64

Logs and stack traces