[tlbuild] libpng16

[Karl Berry]

Hi Elad - the TL source tree includes libpng 1.6.37. We don't make any
changes to the sources, as far as I know. The symbols are not prefixed
with png16_, just png_ as usual.

Thus I wonder if your distro made this change. Which clearly immediately
renders it incompatible with any code using the standard libpng, so it
would seem a strange thing to do, but who am I to say.

The /usr/lib64/libpng.so on my CentOS7 has png_*, FWIW.

As for getting unresolved png16_* symbols when trying to use the libpng
in TL, I believe that is indeed our (build system) bug. From another
person's report, I think our libgd is always using the system libpng,
even when asked not to.  Lacking other contributors with time and energy
to look at the problem, I'll get to it as soon as I can, which won't be
exceptionally soon :(, so I'm glad you can kludge in workarounds.

Sorry I don't have a more useful answer. --best, karl.