MinGW compilation warnings in libiberty's xstrndup.c

[Eli Zaretskii <eliz@gnu.org>]

When compiling libiberty (as part of GDB) with MinGW on MS-Windows, I
see the following warning:

     gcc -c -DHAVE_CONFIG_H -O2 -gdwarf-4 -g3 -D__USE_MINGW_ACCESS  -I. -I./../include   -W -Wall -Wwrite-strings -Wc++-compat -Wstrict-prototypes -pedantic  -D_GNU_SOURCE ./xstrndup.c -o xstrndup.o
     ./xstrndup.c: In function 'xstrndup':
     ./xstrndup.c:51:16: warning: implicit declaration of function 'strnlen' [-Wimplicit-function-declaration]
	size_t len = strnlen (s, n);
		     ^

This happens because libiberty.h uses incorrect guards for the
prototype of strnlen:

  #if defined (HAVE_DECL_STRNLEN) && !HAVE_DECL_STRNLEN
  extern size_t strnlen (const char *, size_t);
  #endif

It should use HAVE_STRNLEN instead, because that's the only
strnlen-related macro defined in config.g when strnlen is probed by
the configure script.

Thanks.