[libpng16] Added #ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED/#endif in pngpread.c
[libpng16] Reduced scope of recently-added #ifdef block

(#ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED/#endif in pngpread.c)
[libpng16] Changed ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED to

ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTE in pngpread.c to agree with
what is in pngpriv.h
[libpng16] Moved prototype for png_handle_unknown() in pngpriv.h outside of

the #ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTED/#endif block.
Removed recently-added #ifdef from pngpread.c; moving the prototype
in pngpriv.h fixed the compiler warning that the #ifdef was trying
to solve.
[libpng16] Added "-Wall" to CFLAGS in contrib/pngminim/*/makefile
[libpng16] Reverted change of ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED

to PNG_SET_UNKNOWN_SUPPORTED in pngpread.c; it led to the possibility of
calling png_handle_unknown() twice.
[libpng16] Conditionally compile some unused functions reported by -Wall

in pngminim.