[tlbuild] trial builds for TL'13

[Karl Berry]

        gcc: web2c-lexer.c: No such file or directory

Well, taking a wild guess, it's the same problem -- the update to the
*.m4 files has caused web2c-lexer.c (generated with (f)lex from
web2c-lexer.l) to be considered out of date, and evidently you don't
have flex (and shouldn't need it).  ylwrap should never be getting
executed.

I just committed a new web2c-lexer.c with a trivial change (r29506) to
update its timestamp.  If it doesn't help, please run make -d in your
Work/texk/web2c/web2c directory and send me the output.

Thanks,
Karl