make sure to undefine "register" (c++11 compatibility)

Parser/Gmsh.l

@@ -63,6 +63,10 @@ void   skipline(void);
 #define YY_NO_UNISTD_H
 #endif
 
+// undefine register for C++11 compatibility of files generated with old
+// versions of flex/bison
+#define register
+
 %}
 
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