autoconf-2.57d regression: AC_HELP_STRING is broken

[Bruno Haible]

Hi,

libtool 1.5 uses AC_HELP_STRING, and with autoconf-2.57d this macro
now produces an empty string instead of an explanation of the option.

How to reproduce:

$ mkdir testdir
$ cd testdir
$ cp .../libtool-1.5/libtool.m4 .
$ cat <<EOF > configure.ac
AC_INIT(configure.ac)
AC_PROG_LIBTOOL
EOF
$ aclocal -I .
$ autoconf
$ ./configure --help
...
Optional Features:
  --disable-FEATURE       do not include FEATURE (same as --enable-FEATURE=no)
  --enable-FEATURE[=ARG]  include FEATURE [ARG=yes]
  --enable-shared[=PKGS]

  --enable-static[=PKGS]

  --enable-fast-install[=PKGS]

  --disable-libtool-lock

Optional Packages:
  --with-PACKAGE[=ARG]    use PACKAGE [ARG=yes]
  --without-PACKAGE       do not use PACKAGE (same as --with-PACKAGE=no)
  --with-gnu-ld
  --with-pic
  --with-tags[=TAGS]

...


PS: Don't tell me that AC_HELP_STRING is obsolete and should be replaced
with AS_HELP_STRING. The announcement mentioned that autoconf-2.58 shall
be released _before_ libtool-1.6. When you make a macro obsolete, it'd
good if it would keep working for a year or two, to avoid constraints
like "libtool 1.5 works only with autoconf <= 2.57, and libtool 1.6 works
only with autoconf >= 2.58".

Bruno