[tlbuild] Bug in libpng affecting Solaris 10

[Akira Kakuto]

Dear Mojca,

> This problem seems to be back.
>     https://sourceforge.net/p/libpng/bugs/245/
>
> I didn't investigate the details yet.

I think that your patch was adopted upstream,
and it is still in the original source.
Please see near line 117 and below in the original
configure.ac. Sorry, if I am wrong.

Thanks,
Akira