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/sasl.c

@@ -24,10 +24,17 @@
 #include "sha1.h"
 #include "scram.h"
 #include "rand.h"
+#include "util.h"
 
-#ifdef _WIN32
-#define strtok_r strtok_s
-#endif
+/* strtok_s() has appeared in visual studio 2005.
+   Use own implementation for older versions. */
+#ifdef _MSC_VER
+# if (_MSC_VER >= 1400)
+# define strtok_r strtok_s
+# else
+# define strtok_r xmpp_strtok_r
+# endif
+#endif /* _MSC_VER */
 
 
 /** generate authentication string for the SASL PLAIN mechanism */