[BUG FIX] gettext fails to build on platforms with stpncpy defined as a complex macro

[Jeremy Huddleston]

On systems where stpncpy is declared as a complex macro, the test will fail.  
This change address the problem:

diff -Naurp gettext-0.18.1.1.orig/gettext-tools/gnulib-m4/stpncpy.m4 
gettext-0.18.1.1/gettext-tools/gnulib-m4/stpncpy.m4
--- gettext-tools/gnulib-m4/stpncpy.m4  2010-05-24 03:42:20.000000000 -0700
+++ gettext-tools/gnulib-m4/stpncpy.m4  2010-08-13 23:23:44.000000000 -0700
@@ -32,7 +32,9 @@ AC_DEFUN([gl_FUNC_STPNCPY],
 #include <stdlib.h>
 #include <string.h> /* for strcpy */
 /* The stpncpy prototype is missing in <string.h> on AIX 4.  */
+#ifndef stpncpy
 extern char *stpncpy (char *dest, const char *src, size_t n);
+#endif
 int main () {
   const char *src = "Hello";
   char dest[10];