[tlbuild] minimal .so dependencies for xetex

[Karl Berry]

Regarding this old mail ...

    Date: Fri, 28 Aug 2009 10:18:25 +0200
    From: Norbert Preining <preining at logic.at>
    To: tlbuild at tug.org
    Subject: Re: [tlbuild] linking against expat ... why

    ...
    This way I got a xetex binary from Martin for x86_64-linux that I will
    commit in a minute that is only linked against
    ...

Peter has implemented this in the sources (a while ago).  It's enabled
for all platforms now.  If it doesn't achieve the right results, let us
know.  If it causes problems, it can be turned off with
--disable-libtool-hack.

thanks,
k