MS-DOS port broken again

[Po Lu <luangruo@yahoo.com>]

After setting up a system to build the MS-DOS port automatically every
two months (which I'm quite proud of), it's broken again:

../lib/libgnu.a(regex.o): In function `build_wcs_buffer':
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:238: undefined reference to `_mbrtowc'
../lib/libgnu.a(regex.o): In function `build_wcs_upper_buffer':
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:382: undefined reference to `_mbrtowc'
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:385: undefined reference to `_towupper'
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:390: undefined reference to `_wcrtomb'
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:382: undefined reference to `_mbrtowc'
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:296: undefined reference to `_mbsinit'
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:312: undefined reference to `_mbrtowc'
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:300: undefined reference to `_towupper'
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:317: undefined reference to `_towupper'
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:322: undefined reference to `_wcrtomb'
../lib/libgnu.a(regex.o): In function `re_string_context_at':
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:916: undefined reference to `_iswalnum'
../lib/libgnu.a(regex.o): In function `re_string_reconstruct':
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:735: undefined reference to `_mbrtowc'
../lib/libgnu.a(regex.o): In function `re_string_skip_chars':
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:503: undefined reference to `_mbrtowc'
../lib/libgnu.a(regex.o): In function `re_string_reconstruct':
c:/djgpp/emacs-29.0.50/lib/regex_internal.c:755: undefined reference to `_iswalnum'
../lib/libgnu.a(regex.o): In function `re_compile_fastmap_iter':
c:/djgpp/emacs-29.0.50/lib/regcomp.c:309: undefined reference to `_mbrtowc'
c:/djgpp/emacs-29.0.50/lib/regcomp.c:367: undefined reference to `_mbrtowc'
c:/djgpp/emacs-29.0.50/lib/regcomp.c:381: undefined reference to `_wcrtomb'
c:/djgpp/emacs-29.0.50/lib/regcomp.c:385: undefined reference to `_towlower'
c:/djgpp/emacs-29.0.50/lib/regcomp.c:385: undefined reference to `_wcrtomb'
c:/djgpp/emacs-29.0.50/lib/regcomp.c:309: undefined reference to `_towlower'
c:/djgpp/emacs-29.0.50/lib/regcomp.c:309: undefined reference to `_wcrtomb'
../lib/libgnu.a(regex.o): In function `peek_token':
c:/djgpp/emacs-29.0.50/lib/regcomp.c:1892: undefined reference to `_iswalnum'
c:/djgpp/emacs-29.0.50/lib/regcomp.c:1781: undefined reference to `_iswalnum'
../lib/libgnu.a(regex.o): In function `parse_byte':
c:/djgpp/emacs-29.0.50/lib/regcomp.c:2627: undefined reference to `_btowc'
../lib/libgnu.a(regex.o): In function `build_charclass':
c:/djgpp/emacs-29.0.50/lib/regcomp.c:3481: undefined reference to `_wctype'
../lib/libgnu.a(regex.o): In function `re_compile_internal':
c:/djgpp/emacs-29.0.50/lib/regcomp.c:873: undefined reference to `_btowc'
../lib/libgnu.a(regex.o): In function `check_node_accept_bytes':
c:/djgpp/emacs-29.0.50/lib/regexec.c:3713: undefined reference to `_iswctype'

It seems as if gnulib gained a hard dependency on mbrtowc, towupper, and
other related functions.

The rest of these functions can be defined as macros, but I have no idea
what to do with mbrtowc, nor can I find any documentation on the format
of shift state accepted by DJGPP.

Any ideas?  Thanks in advance.