fix: define S_ISREG on Windows

[LaplaceSoda]

• detect whether POSIX-style macros exist before using them
• fall back to the MSVC _S_IF* constants when needed
• to fix Build error with ifx on Windows #1060

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.14%. Comparing base (513ff50) to head (bb86830).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1061      +/-   ##
==========================================
- Coverage   25.14%   25.14%   -0.01%     
==========================================
  Files         570      570              
  Lines      234225   234225              
  Branches    41283    41285       +2     
==========================================
- Hits        58905    58903       -2     
- Misses     175320   175322       +2     

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[perazz]

LGTM @LaplaceSoda, thank you. Can I ask you to remove the style/format changes, so this PR better highlights the new preprocessor definitions?

[LaplaceSoda]

LGTM @LaplaceSoda, thank you. Can I ask you to remove the style/format changes, so this PR better highlights the new preprocessor definitions?

Done! I've removed the style/format changes and kept only the essential preprocessor definitions. Should be much clearer now.

[perazz]

Will wait a bit longer and then merge, if there are no further comments. Thank you @LaplaceSoda.

[jvdp1]

thank you @LaplaceSoda . LGTM.

[jalvesz]

LTGM @LaplaceSoda, thanks for this contribution!