flex-2.6.3 compatibility fix
flex-2.6.3 changed its handling of yywrap(). The new behavior always defines the yywrap macro. Therefore, the yywrap() function needs to be defined irrespective of whether the yywrap macro is defined.
This patch updates lexer.l so that it defines the yywrap() function when flex-2.6.3 or newer is used. Otherwise, it reverts to the previous behavior. It defines the yywrap() function only if the yywrap macro is not defined.