[tlbuild] libpng16

[Elad Lahav]

Hello,

I'm building texlive from source under QNX, and I'm running into
problems with libpng. The system provides libpng16, but I cannot build
with --use-system-libpng. Apparently the API functions in this version
have all been renamed from png_* to png16_*, and so the 'configure'
script fails. On the other hand, if I try to build with the included
version of libpng then web2c fails with unresolved png16_* symbols.
I can hack my way around these issues, but I assume that other systems
also have libpng16 only, so hopefully there is already a solution to
this problem.

Thanks,
--Elad