[libiberty] Make test for variables more robust

[Eric Botcazou]

Hi,

the test for the presence of variables (really symbols) does not work when you 
add -Ox -flto to CFLAGS:

  for v in $vars; do
    AC_MSG_CHECKING([for $v])
    AC_CACHE_VAL(libiberty_cv_var_$v,
      [AC_LINK_IFELSE([AC_LANG_PROGRAM([[int *p;]],[[extern int $v []; p = 
$v;]])],
                      [eval "libiberty_cv_var_$v=yes"],
                      [eval "libiberty_cv_var_$v=no"])])
    if eval "test \"`echo '$libiberty_cv_var_'$v`\" = yes"; then
      AC_MSG_RESULT(yes)
      n=HAVE_`echo $v | tr 'abcdefghijklmnopqrstuvwxyz' 
'ABCDEFGHIJKLMNOPQRSTUVWXYZ'`
      AC_DEFINE_UNQUOTED($n)
    else
      AC_MSG_RESULT(no)
    fi
  done

because the assignment to 'p' is optimized away by LTO.  This is visible on 
MinGW platforms in the form of a link failure for sys_siglist.

Tested on x86-64/Linux, x86-64/Windows and x86/Windows, OK for the mainline?


2021-05-03  Eric Botcazou  <ebotca...@adacore.com>

        * configure.ac: Make test for variables more robust.
        * configure: Regenerate.

-- 
Eric Botcazou
diff --git a/libiberty/configure.ac b/libiberty/configure.ac
index ad952963971..3c03f24d368 100644
--- a/libiberty/configure.ac
+++ b/libiberty/configure.ac
@@ -665,7 +665,7 @@ if test -z "${setobjs}"; then
   for v in $vars; do
     AC_MSG_CHECKING([for $v])
     AC_CACHE_VAL(libiberty_cv_var_$v,
-      [AC_LINK_IFELSE([AC_LANG_PROGRAM([[int *p;]],[[extern int $v []; p = $v;]])],
+      [AC_LINK_IFELSE([AC_LANG_PROGRAM([[extern int $v [];]],[[if ($v [0]) return 1;]])],
 		      [eval "libiberty_cv_var_$v=yes"],
 		      [eval "libiberty_cv_var_$v=no"])])
     if eval "test \"`echo '$libiberty_cv_var_'$v`\" = yes"; then