$ libpng-config --L_opts
$ libpng-config --R_opts
This is quite inconsistent, unnecessary and when applied with bug 487696, causes failures for multilib.
While I can agree there are other ways of solving this, IMO just dropping the -L flag would be the simplest one. That's what pkg-config does by default, for example.
if your package is still using libpng-config, it is broken. i say use this as a marker for "i need to update to pkg-config".