util: added strtok_r implementation for old compilers

Visual studios older than 2005 don't have strtok_s() and according to
MSDN vs2005 has NOT thread-safe strtok().

src/util.c

@@ -55,6 +55,28 @@ char *xmpp_strdup(const xmpp_ctx_t * const ctx, const char * const s)
     return copy;
 }
 
+/** strtok_r(3) implementation.
+ *  This function has appeared in POSIX.1-2001, but not in C standard.
+ *  For example, visual studio older than 2005 doesn't provide strtok_r()
+ *  nor strtok_s().
+ */
+char *xmpp_strtok_r(char *s, const char *delim, char **saveptr)
+{
+    size_t len;
+
+    s = s ? s : *saveptr;
+    len = strspn(s, delim);
+    s += len;
+    if (*s == '\0')
+        return NULL;
+
+    len = strcspn(s, delim);
+    *saveptr = s[len] == '\0' ? &s[len] : &s[len + 1];
+    s[len] = '\0';
+
+    return s;
+}
+
 /** Return an integer based time stamp.
  *  This function uses gettimeofday or timeGetTime (on Win32 platforms) to
  *  compute an integer based time stamp.  This is used internally by the