Use a guard macro to prevent the inclusion of system headers

when checking the ABI with the C Preprocessor. Also, add the
new hearders to the cmake based ABI check.

include/mysql/client_plugin.h

@@ -23,8 +23,10 @@
 */
 #define MYSQL_CLIENT_PLUGIN_INCLUDED
 
+#ifndef MYSQL_ABI_CHECK
 #include <stdarg.h>
 #include <stdlib.h>
+#endif
 
 /* known plugin types */
 #define MYSQL_CLIENT_reserved1               0