obsolete code: libpng-config

[jbowler]

libpng-config (libpng-config, libpng16-config) and pkgconfig (libpng.pc, libpng16.pc) provide duplicate functionality however libpng-config is a traditional "by hand" shell script whereas libpng.pc is a data file which just contains the relevant information.

Apparently the -config approach is obsolate and libpng16-config does not work in some environments, see the comments here:

https://bugs.gentoo.org/487740

I suggest libpng-config and libpng16-config be removed from the install (so build them but don't install them) to find out if anyone really uses them then either remove them in libpng16 or remove them if there are currently used in libpng18.

[ctruta]

While I agree with you about not making changes that are unnecessarily large and/or gratuitously disruptive in the libpng16 branch (which is, in general, the way to go about applying changes to software in maintenance mode), we should absolutely clean this up in libpng18.