needed by png_reciprocal2().
Added #ifdef PNG_16BIT_SUPPORTED/#endif around png_log16bit() and
    png_do_swap().

in the manual, example.c, pngtest.c, and applications in the contrib directory.
It was incorrect advice.

in the code the number of elements in an array is calculated using
sizeof.  This generates a compile time constant of type (size_t) which
is then typically assigned to an (unsigned int) or (int). Some versions
of GCC on 64-bit systems warn about the apparent narrowing, even though
the same compiler does apparently generate the correct, in-range,
numeric constant.  This adds appropriate, safe, casts to make the
warnings go away.

allocated at the end of the program, harmless but it causes memory
leak reports from clang.

seem to generate warnings when an unsigned value is implicitly
converted to double. This is probably a GCC bug but this change
avoids the issue by explicitly converting to (int) where safe.

revision to configure.ac to support ARM on aarch64 instead (John Bowler).

"if (x !== 0)"

in the manual, to reflect the change made in libpng-1.6.0.
Updated README file to reflect that direct access to the info struct and
png struct are not permitted since libpng-1.5.0.