Errors fixed:
Not detected by the compiler, but the preprocessor version checks only
looked at `CYGWIN_VERSION_API_MINOR`, ignoring
`CYGWIN_VERSION_API_MAJOR`.
    cygwin.c: In function 'utf8_to_wide_extra_len':
    cygwin.c:187:18: warning: initialization of 'size_t' {aka 'long unsigned int'} from 'size_t *' {aka 'long unsigned int *'} makes integer from pointer without a cast [-Wint-conversion]
      187 |     Size_t len = strlen(buf) + extra_len + 1;
          |                  ^~~~~~
`extra_len` was declared as a pointer, but used as an integer. I don't
see how this could have ever worked.
    cygwin.c: In function 'S_convert_path_common':
    cygwin.c:339:38: warning: 'realloc' called on unallocated object 'wconverted' [-Wfree-nonheap-object]
      339 |             wconverted = (wchar_t *) realloc(&wconverted, newlen);
          |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
    cygwin.c:357:39: warning: 'realloc' called on unallocated object 'converted_path' [-Wfree-nonheap-object]
      357 |             converted_path = (char *) realloc(&converted_path, newlen);
          |                                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Again, I don't see how this could have ever worked. We're passing the
address of a local (stack) variable to `realloc` (instead of the value
of the variable).
    cygwin.c: In function 'wide_to_utf8':
    cygwin.c:173:32: warning: pointer targets in passing argument 3 of 'Perl_utf16_to_utf8_base' differ in signedness [-Wpointer-sign]
      173 |     utf16_to_utf8((U8 *) wsrc, buf, wlen, &blen);
          |                                ^~~
          |                                |
          |                                char *
The argument is declared as `U8 *`, so this is mostly harmless, but a
cast silences the warning anyway.
    cygwin.c: In function 'utf8_to_wide_extra_len':
    cygwin.c:194:19: warning: passing argument 2 of 'Perl_utf8_to_utf16_base' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
      194 |     utf8_to_utf16(buf, (U8 *) wsrc, len, &wlen);
          |                   ^~~
This is arguably a bug in `utf8_to_utf16`, which doesn't declare its
input argument as pointer-to-const. But it's also a `char *` vs `U8 *`
type mismatch, so a cast silences both issues.
    cygwin.c: In function 'S_convert_path_common':
    cygwin.c:292:22: warning: initialization discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
      292 |         char *name = (direction == to_posix)
          |                      ^
This is a missing `const` in the declaration of a variable that points
to string literals.