XSUB.h: fix C23 incompatibility in XSINTERFACE_* macros

[xenu]

In C23, the meaning of an empty function prototype has changed. It used to mean "unspecified number of arguments", but it has now changed to "no arguments".

Fixes #23192

[jkeenan]

In C23, the meaning of an empty function prototype has changed. It used to mean "unspecified number of arguments", but it has now changed to "no arguments".

Do you anticipate a similar change in clang?

[xenu]

So far, clang hasn't bumped the standard C version (they're still at C17), but it's likely they will at some point in the future.

If you use clang >=18, you can force it by configuring Perl with -Accflags=-std=gnu23. Building CommonMark should fail in the same way as in the bug report, and my PR should fix it.

GCC 14 would also fail with this flag, but unfortunately my fix doesn't work for this particular version, because GCC 14 does not set __STDC_VERSION__ properly. As this is a rather unusual configuration, I don't think working around it would be worth it.

[Leont]

I'm not sure this fix is right (I have to dig deeper for that), but I am very sure that the implementation of the INTERFACE feature is rather broken and should probably be fixed.

xsubpp knows the types of the arguments of the function, it should be able to cast to the right type without needing this silly macro.

[xenu]

While it being UB isn't very surprising, I thought it's at least one of those things that work in practice. And it is on most platforms, but apparently not on macOS on Apple Silicon, which uses a special ABI for vararg functions.

Oh well, I guess we will have to fix it the hard way.

[thesamesam]

Right, calling it via ... isn't OK unless the real function genuinely is variadic. Definitely was broken for C++ before.