[tlbuild] Build script "improvements"

[Karl Berry]

Hi Olaf,

    A peculiarity of the IRIX build is that I have to do some fixups after
    the configure run 

Can we fix the actual problems needing the "fixups"?

      ./Build --no-clean --no-configure --no-make
    Is this considered worth checking in?

I don't mind in principle, but in detail:

1) it's time to switch the option parsing to the usual case..esac
   instead of the kludgy $1 tests.

2) I find the double negatives for the various (TL_NO_{CONFIGURE,MAKE})
   confusing.  How about making them positive (TL_DO_...).

3) I wonder about using "true" instead of separate flags with 1/0
   flags that have to be tested.  I mean, to get --no-configure, set
   TL_CONFIGURE to "true".

Wdyt?

Anyway, feel free to check in what you want and I'll munge it if I feel
the need :).

Thanks,
k