[Win32] Define $Config{i_stdckdint} and 'I_STDCKDINT' when appropriate

[sisyphus]

This PR , in conjunction with (the already merged) #23703, will define $Config{i_stdckdint} and I_STDCKDINT on Windows builds of perl whenever stdckdint.h is locatable and functional.

It currently applies only to mingw-built perls. (Microsoft compilers do not yet provide stdckdint.h and I could find nothing to suggest that they ever will.)

Hence, with this PR. stdckdint.h will be included only if perl on Windows is built using gcc-14 or later && $Config{cc} is gcc.
(I don't know if anyone builds perl on Windows with g++, but according to my testing stdckdint.h is non-functional on Windows when the compiler is g++.)

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• This set of changes does not require a perldelta entry.

[tonycoz]

(Microsoft compilers do not yet provide stdckdint.h and I could find nothing to suggest that they ever will.)

cppreference claims C++26 will require stdckdint.h so I expect they will add it.

[sisyphus]

Not sure what I've stuffed up here.
Am I supposed to wait until the CI-tests have completed before squashing the commits ?
Guidance on how to proceed is welcome.

UPDATE:
Hmmm ... win32/configure/have_stdckdint.c has apparently gone AWOL.

[khwilliamson]

You can squash whenever you like; CI will start over (I'm unsure if it continues with the unsquashed version)

[sisyphus]

You can squash whenever you like; CI will start over (I'm unsure if it continues with the unsquashed version)

Thanks Karl.
I've obviously squashed more than just the commit messages.
I'll close this and redo it as a new PR.