[libpng12] Updated copyright year in recently-changed files.
[libpng12] Rebuilt configure scripts with autoconf-2.69 and automake-1.14.1
[libpng12] Updated copyright year in png.c.
[libpng12] Imported from libpng-1.2.51beta03.tar
[libpng12] Bump version to 1.2.51beta04