[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.

On 2017-12-02, Karl added
if false; then # line 29
...
fi

in our libpng/configure.ac.
So your patch seems to be killed. Karl?

Best,
Akira