Avoid warning about yywrap being redefined.

Avoid warning about yywrap being redefined.

The flex documentation says

	If you do not supply your own version of 'yywrap()', then you must
	either use '%option noyywrap' (in which case the scanner behaves as
	though 'yywrap()' returned 1), or you must link with '-lfl' to obtain
	the default version of the routine, which always returns 1.

Unfortunately, if noyywrap is given, gcc-5.4.0 complains with

	config_file.c:417:0: warning: "yywrap" redefined

	 ^
	config_file.c:74:0: note: this is the location of the previous definition

Just provide our own yywrap() to silence the warning.