Fix an issue with the IBM C++ compiler

which was detected during the build of 5.5.3-m3.
This requires version 9 of IBM C++ to help.

More fixes will still be needed, it is very strict
(or rather: a bit picky?) about templates.

sql/mdl.h

@@ -15,6 +15,14 @@
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
 
+#if defined(__IBMC__) || defined(__IBMCPP__)
+/* Further down, "next_in_lock" and "next_in_context" have the same type,
+   and in "sql_plist.h" this leads to an identical signature, which causes
+   problems in function overloading.
+*/
+#pragma namemangling(v5)
+#endif
+
 
 #include "sql_plist.h"
 #include <my_sys.h>