$ echo $LD_LIBRARY_PATH 
/home/me/local/lib

$ flex --version
flex 2.6.0

$ cat fb1-1.l 
/* fb1-1 just like unix wc */
/* the "%option noyywrap" was added to fix the error, but it just changed it. The LD error is kept. */
%{
int chars = 0;
int words = 0;
int lines = 0;
%}

%option noyywrap

%%

[a-zA-Z]+	{ words++; chars += strlen(yytext); }
\n		{ chars++; lines++; }
.		{ chars++; }

%%

int main()
{
  yylex();
  printf("%8d%8d%8d\n", lines, words, chars);
  return 0;
}

$ flex fb1-1.l 

$ # this is a wrong gcc compilation, we must link the file with flex library

$ gcc -Wall lex.yy.c 
lex.yy.c:1104: warning: ‘yyunput’ defined but not used
lex.yy.c:1149: warning: ‘input’ defined but not used

$ # but it worked?!

$ # now the right one, like the book instructs to do

$ gcc lex.yy.c -lfl

lex.yy.c:1104: warning: ‘yyunput’ defined but not used
lex.yy.c:1149: warning: ‘input’ defined but not used
/usr/bin/ld: cannot find -lfl
collect2: ld returned 1 exit status

$ # the installed flex libraries?

$ 'ls' /home/me/local/lib
libfl.a      libfl_pic.la    libfl_pic.so.2.0.0  libfl.so.2.0.0
libfl.la     libfl_pic.so    libfl.so		 liby.a
libfl_pic.a  libfl_pic.so.2  libfl.so.2

$ echo $LD_LIBRARY_PATH 
/home/me/local/lib