(HAVE_STRING_H): Add #undef.

[HAVE_STRING_H]: Include string.h.

src/config.in

@@ -25,7 +25,6 @@ Boston, MA 02111-1307, USA.  */
 #ifndef EMACS_CONFIG_H
 #define EMACS_CONFIG_H
 
-
 /* These are all defined in the top-level Makefile by configure.
    They're here only for reference.  */
 
@@ -131,6 +130,7 @@ Boston, MA 02111-1307, USA.  */
 #undef HAVE_SYS_SYSTEMINFO_H
 #undef HAVE_TERMIOS_H
 #undef HAVE_LIMITS_H
+#undef HAVE_STRING_H
 #undef STDC_HEADERS
 #undef TIME_WITH_SYS_TIME
 
@@ -404,3 +404,7 @@ extern char *getenv ();
 #ifndef BITS_PER_LONG
 #define BITS_PER_LONG 32
 #endif
+
+#ifdef HAVE_STRING_H
+#include "string.h"
+#endif