Fixing __has_include(<stdckdint.h>) compilation error

[darkain]

The very thing the conditional for __has_include is trying to handle, is causing compilation to break.

This file was brought into the ruby/bigdecimal project, and is failing to compile on a platform which doesnt have __has_include support in the compiler.

Compile log can be found in this comment:
ruby/bigdecimal#531 (comment)

[zenspider]

This should be ported over to bigdecimal and any other libs that might use this file.

[nobu]

What compiler and environment?

[darkain]

@nobu This is on Oracle Linux 7.9

[nobu]

Thank you.
What is the exact compiler version?

I found that the default compiler on Oracle Linux 7 is gcc 4.8.5, but it is too old.
I think you install a newer version.

[darkain]

The specific check in the code that I'm submitting this PR for is explicitly a fix for "old compilers" that wasn't done correctly. Its the only thing that prevents the code from working on these systems.

[tompng]

GCC 5 supports __has_include, so your GCC version is 4.8.5 ~ 4.9.4. Ruby >= 3.3 dropped support of GCC <= 6 (https://bugs.ruby-lang.org/issues/19353), and 3.2 is already EOL. So the code is correct for the oldest supported GCC version in the oldest supported Ruby version.

It's unacceptable to merge this as a fix, but I think it's OK as a refactor if it's refactored.

Comparing empty-if and elif vs 3-depth nested if, it's hard to say that it's clearly improved. Can you change the 3-depth nested if to 2-depth nested if like this?

#if !defined(HAVE_STDCKDINT_H) && defined(__has_include)
#if __has_include(<stdckdint.h>)