[Bug libstdc++/109818] std::trunc() requires a hack after building DJGPP

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109818

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Also:

  if test $enable_cheaders = c_std ; then
    AC_MSG_WARN([the --enable-cheaders=c_std configuration is obsolete,
c_global should be used instead])
    AC_MSG_WARN([if you are unable to use c_global please report a bug or
inform libstd...@gcc.gnu.org])
    if test $enable_cheaders_obsolete != yes ; then
      AC_MSG_ERROR(use --enable-cheaders-obsolete to use c_std "C" headers)
    fi
  fi

So somebody is ignoring that when configuring djgpp. I think it would be much
better to figure out how to get djgpp using the new header instead.