[tlbuild] trial builds for TL'13

[Karl Berry]

    > 2. GNU sed is required. Setting

I see nothing GNU-sed-specific in there.  Is there a specific construct
being used that you know BSD sed does not support?

    when it has to be stopped with Ctrl+C.

It's hard to be completely sure, but from the description, I suspect the
problem is that $< is being used in a non-implicit rule, and expanding
to nothing, and thus sed is waiting for input.  At least, I don't see
paths.h being used as an input file.

Which further implies that GNU make is not being used for that
particular part (which puzzles me greatly), since GNU make supports $<
in regular rules.  Regardless, though, that's neither intentional nor
desirable and I'll look into it.

Thanks,
k