fix for strndup on AIX 5.1

[Bruno Haible]

Matthew Woehlke reported an strndup related compilation error on AIX 5.1
on bug-coreutils. This fixes it.

2007-03-20  Bruno Haible  <address@hidden>

        * lib/string_.h (strndup): Enable declaration also if ! HAVE_STRNDUP.
        Needed on AIX 5.1. Reported by Matthew Woehlke.

*** lib/string_.h       21 Feb 2007 00:02:37 -0000      1.28
--- lib/string_.h       21 Mar 2007 04:18:33 -0000
***************
*** 192,200 ****
  # if ! @HAVE_STRNDUP@
  #  undef strndup
  #  define strndup rpl_strndup
! #  if ! @HAVE_DECL_STRNDUP@
  extern char *strndup (char const *__string, size_t __n);
- #  endif
  # endif
  #elif defined GNULIB_POSIXCHECK
  # undef strndup
--- 192,200 ----
  # if ! @HAVE_STRNDUP@
  #  undef strndup
  #  define strndup rpl_strndup
! # endif
! # if ! @HAVE_STRNDUP@ || ! @HAVE_DECL_STRNDUP@
  extern char *strndup (char const *__string, size_t __n);
  # endif
  #elif defined GNULIB_POSIXCHECK
  # undef strndup