[RIOT] coap_config.h.riot: undef HAVE_STRNLEN

strnlen() is not available on all RIOT boards, therefore the
substitution in libcoap is used for now.

coap_config.h.riot

@@ -31,7 +31,7 @@
 
 /* Define to 1 if your system has a GNU libc compatible `malloc' function, and
    to 0 otherwise. */
-#define HAVE_MALLOC 0
+#undef HAVE_MALLOC
 
 /* Define to 1 if you have the <memory.h> header file. */
 /* #undef HAVE_MEMORY_H */
@@ -67,7 +67,7 @@
 #define HAVE_STRING_H 1
 
 /* Define to 1 if you have the `strnlen' function. */
-#define HAVE_STRNLEN 1
+/* #undef HAVE_STRNLEN */
 
 /* Define to 1 if you have the `strrchr' function. */
 #define HAVE_STRRCHR 1