The scanner calls this function on end of file, so you can point it to another file and continue scanning its contents. If you don’t need this, use
%option noyywrap
in the scanner specification.
Although disabling yywrap is certainly the best option, it may also be possible to link with -lfl to use the default yywrap() function in the library fl (i.e. libfl.a) provided by flex. Posix requires that library to be available with the linker flag -ll and the default OS X install only provides that name.