MDEV-8285 compile fails under Mac OS X 10.6.8 due to use of strnlen

#include <m_string.h> where strnlen() is used

storage/connect/xobject.cpp

@@ -11,6 +11,7 @@
 /*  Include mariaDB header file.                                       */
 /***********************************************************************/
 #include "my_global.h"
+#include "m_string.h"
 
 /***********************************************************************/
 /*  Include required application header files                          */

strings/CMakeLists.txt

@@ -26,7 +26,7 @@ SET(STRINGS_SOURCES bchange.c bmove_upp.c ctype-big5.c ctype-bin.c ctype-cp932.c
 		my_strchr.c strcont.c strappend.c)
 
 IF(NOT HAVE_STRNLEN)
-  # OSX does not have strnlen
+  # OSX below 10.7 did not have strnlen
   SET(STRINGS_SOURCES ${STRINGS_SOURCES} strnlen.c)
 ENDIF()
 # Avoid dependencies on perschema data defined in mysys