[libpng17] Added #ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED/#endif in pngpread.c
[libpng17] Moved prototype for png_handle_unknown() in pngpriv.h outside of

the #ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTED/#endif block.
[libpng17] Added "-Wall" to CFLAGS in contrib/pngminim/*/makefile