[libfko] use local strndup() if autoconf HAVE_STRNDUP not defined

Blair Zajac reported that strndup() is not available on some PPC systems, so
this commit switches to use the local lib/fko_util.c implementation similarly
to what is done for Windows systems.

lib/fko_util.c

@@ -399,11 +399,11 @@ strtol_wrapper(const char * const str, const int min,
 }
 
 
-#ifdef WIN32
+#if defined(WIN32) || !defined(HAVE_STRNDUP)
 /* Windows does not have strndup, so we well implement it here.
  * This was the Public Domain C Library (PDCLib).
 */
-char 
+char
 *strndup( const char * s, size_t len )
 {
     char* ns = NULL;

lib/fko_util.h

@@ -52,7 +52,7 @@ const char * msg_type_inttostr(const int type);
 size_t  strlcat(char *dst, const char *src, size_t siz);
 size_t  strlcpy(char *dst, const char *src, size_t siz);
 
-#ifdef WIN32
+#if defined(WIN32) || !defined(HAVE_STRNDUP)
 char * strndup( const char * s, size_t len );
 #endif