$ libpng-config --L_opts -L/usr/lib64 $ libpng-config --R_opts (none) 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".