AC_PROG_LEX behavior

[Stefan Seefeld]

I'm trying to use AC_PROG_LEX to discover whether flex (or lex) is 
available. During testing I found out that, even when neither is 
available, the LEX variable is set to ':' instead of being empty.
And sure enough, I subsequently found this in the programs.m4 code:


AC_DEFUN_ONCE([AC_PROG_LEX],
[AC_CHECK_PROGS(LEX, flex lex, :)
if test "x$LEX" != "x:"; then
  _AC_PROG_LEX_YYTEXT_DECL
fi])

Is this intentional ? Is it documented somewhere ? In other words, is it 
ok for me to check for "test $LEX != ':'" to discover success ?

Thanks,
                Stefan

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