warn-on-use.m4 bug

[Bruno Haible]

Hi Eric,

There seems to be a bug in warn-on-use.m4 when used with autoconf-2.60.
No problem with autoconf-2.61 and newer.

How to reproduce:
  - Put autoconf 2.60 binaries in PATH.
  - Use automake 1.10 or 1.9.6.
  - $ ./gnulib-tool --create-testdir --dir=/tmp/testdir sys_stat
  - $ cd /tmp/testdir
  - $ ./configure
    ...
    checking whether fchmodat is declared without a macro... ./configure: line 
6206: ${$as_gl_Symbol+set}: bad substitution
    checking for a thread-safe mkdir -p... /arch/x86-linux/gnu/bin/mkdir -p
    ...

The expansion in configure looks like this:

  for gl_func in fchmodat fstatat futimens lchmod lstat mkdirat mkfifo mkfifoat 
    mknod mknodat stat utimensat; do
    as_gl_Symbol=`echo "gl_cv_have_raw_decl_$gl_func" | $as_tr_sh`
    { echo "$as_me:$LINENO: checking whether $gl_func is declared without a 
macro" >&5
echo $ECHO_N "checking whether $gl_func is declared without a macro... $ECHO_C" 
>&6; }
if test "${$as_gl_Symbol+set}" = set; then                              <====== 
BUG
  echo $ECHO_N "(cached) $ECHO_C" >&6
else
  cat >conftest.$ac_ext <<_ACEOF
/* confdefs.h.  */
_ACEOF
cat confdefs.h >>conftest.$ac_ext
cat >>conftest.$ac_ext <<_ACEOF
...
  $as_gl_Symbol=yes                                                     <====== 
BUG
else
  echo "$as_me: failed program was:" >&5
sed 's/^/| /' conftest.$ac_ext >&5

        $as_gl_Symbol=no                                                <====== 
BUG
fi

rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
fi
{ echo "$as_me:$LINENO: result: $$as_gl_Symbol" >&5                     <====== 
BUG
echo "${ECHO_T}$$as_gl_Symbol" >&6; }                                   <====== 
BUG
     if test x"$$as_gl_Symbol" = x""yes; then                           <====== 
BUG

As you can see, all uses of $as_gl_Symbol are broken.

Bruno