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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/109818] std::trunc() requires a <cmath> hack after
 building DJGPP
Date: Thu, 11 May 2023 20:13:06 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109818

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

  if test $enable_cheaders =3D c_std ; then
    AC_MSG_WARN([the --enable-cheaders=3Dc_std configuration is obsolete,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 libstdc++@gcc.gnu.org])
    if test $enable_cheaders_obsolete !=3D 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 mu=
ch
better to figure out how to get djgpp using the new header instead.=