[tlbuild] Fixes for libpng building on ARM

[Peter Breitenlohner]

On Wed, 8 Oct 2014, Norbert Preining wrote:

> we got a bug report that the current sources (which are very very
> close to what is in svn) does not build on arm64.
>
> Explanation and a bug fix is attached, thanks to Wookey in Cc.
>
> ...
>
> Maybe it makes sense to have this in our svn, too.

Hi Norbert,

first of all the problem should be reported upstream (if not already done
so).  We certainly could apply the patch to the TL svn, but I'd prefer to
first wait for a reaction and perhaps a better solution from the current
maintainer Glenn Randers-Pehrson.

Regards
Peter