Fix logic error in _AC_PROG_LEX_YYTEXT_DECL (#109320)

Fix logic error in _AC_PROG_LEX_YYTEXT_DECL (#109320)

The search for the appropriate value for @LEXLIB@ did not distinguish
correctly between success (‘break’ out of a shell for loop, having set
$ac_cv_lib_lex) and failure (normal termination of the loop, value in
$ac_cv_lib_lex is garbage).  Bug report and original patch by
Tom <tom@ojodeagua.com> with refinements by Zack Weinberg.

* lib/autoconf/programs.m4 (_AC_PROG_LEX_YYTEXT_DECL):
  Error out if we cannot find the library that (f)lex scanners
  need to be linked against, instead of continuing with @LEXLIB@
  set to the empty string.