The info files state that flex can be used with the option -+
to generate a class yyFlexLexer. The .cc file generated by flex
#includes a file FlexLexer.h, which must also be included by any other file 
calling yyFlexLexer::YYLexer().

I couldn't find this header file anywhere.

A possible workaround is to write up a flexlexer.h file from the description
in info. I haven't tried it yet.