[tlbuild] Broken TL build

[Karl Berry]

    The following commit broke on more than one half of our builders:
        https://tug.org/svn/texlive?view=revision&revision=61619

Well, drat.

        https://build.contextgarden.net/#/waterfall?tags=c.texlive

It is painfully time-consuming for me to extract the actual errors from
your build system's web interface. It is too advanced /
graphics-oriented / javascript-heavy for my world. So, can you just tell
me what the errors actually are? Sorry.

BTW, I am surprised that changing -L. -lweb2c to libweb2c.a makes a
difference.  It seems it should be entirely equivalent.  Sigh.

Of course it is easy to revert if we cannot fix it in a better way
(sorry Michael/slibtool in that case). --thanks, karl.