[libpng17] Imported from libpng-1.7.0beta25.tar
[libpng17] Bump version to 1.7.0beta26
[libpng17] Removed configure-generated "compile" file.
[libpng17] Added libpng 1.5 checks to pngvalid.c
[libpng17] Merged with 1.5 and 1.6 changes to create a single pngvalid.c
[libpng17] Added dSIG to chunk list in png.h