Like TDragon mentioned, the file lex.yy.cc might not exist, you first need to execute the flex tool:
where flexfile.l is your lexer's syntax file (usually suffixed with .l).
Once the compilation of the flex tool is done, you should have the default file name lex.yy.
c Note that the default file is a 'C'. If you want to generate the C++ file, you should include FlexLexer.h, and invoke the flex like this:
BTW, the
is for not including the #line directive in the generated code.
I suggest that you will add some pre build steps to generate lex.yy.c file prior compilation.
HTH,
Eran