No need for code conditional on S_IFMT being defined, as perl.h has a fallback.

Commit c623bd54707a8bf9 (Jan 1991) added code conditionally compiled if
S_IFMT was defined. However, the checks were immediately redundant, because
the same commit also added code to perl.h to define a fallback value for
S_IFMT if the OS headers did not define it, which is unchanged to this day.
Hence the code added in an #else as part of commit 99b89507a1fb507c (Nov
1991) has never ever been needed.