[Bug other/68610] configure failure in libssp

["gccbugzilla at limegreensocks dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68610

--- Comment #2 from David <gccbugzilla at limegreensocks dot com> ---
Yes, this still occurs with trunk Revision 239974.  I'm using MSYS2 under
Windows and my current configure line is:

../gcctrunk/configure --enable-languages=c,c++ --target=x86_64-w64-mingw32
--build=x86_64-w64-mingw32 --host=x86_64-w64-mingw32 --disable-multilib
--disable-nls --disable-win32-registry --enable-checking=release
--disable-bootstrap --enable-version-specific-runtime-libs
--disable-fixed-point --prefix=/home/david/gccb --enable-twoprocess
--with-native-system-header-dir=/mingw64/include

None of which turns out to be important.

Looking at libssp/configure.ac, we see this code for testing to see if
fstack-protector works:

AC_MSG_CHECKING([whether -fstack-protector works])
save_CFLAGS="$CFLAGS"
CFLAGS="$CFLAGS -fstack-protector -Werror"
AC_TRY_COMPILE([
void __attribute__((noinline)) bar (char *x)
{
  __builtin_memset (x, 0, 64);
}],[char buf[64]; bar (buf);],
[AC_MSG_RESULT(yes)],
[AC_MSG_RESULT(no)])
CFLAGS="$save_CFLAGS"

Note how it saves the CFLAGS, explicitly sets Werror, then does the
AC_TRY_COMPILE.  Contrast this with the code that immediately follows which
attempts to do the same thing for visibility:

AC_MSG_CHECKING([whether hidden visibility is supported])
AC_TRY_COMPILE([
void __attribute__((visibility ("hidden"))) bar (void) {}],,
[ssp_hidden=yes],[ssp_hidden=no])
AC_MSG_RESULT($ssp_hidden)
if test x$ssp_hidden = xyes; then
  AC_DEFINE([HAVE_HIDDEN_VISIBILITY],[1],[__attribute__((visibility
("hidden"))) supported])
fi

If visibility is NOT supported, the compiler will produce a warning.  However,
since in this case Werror is NOT being specified, the compile 'succeeds,' and
visibility is assumed to be supported, even though the compiler explicitly said
it wasn't.

I've learned a lot about how configure scripts work since I originally posted
this bug.  Hopefully this helps makes the problem clearer.