outside of the PNG_READ|WRITE_ANCILLARY_CHUNK_SUPPORTED macros and
updated CHANGES and ANNOUNCE.  The definitions were actually relocated
in the previous commit but I forgot to mention it.

to PNG_HANDLE_AS_UNKNOWN to make it possible to build a libpng that can
use the png_set_keep_unknown_chunks() mechanism to ignore unused chunks
without having to save unknown chunks.

about calling png_set_IHDR() multiple times and about the order
of color samples in the tRNS chunk.

about using png_set_io_ptr() in configure scripts for detecting libpng

and "else statement;" into two lines.

Only png_read_finish_row() can be omitted.

which are only used by sequential PNG readers.  This can save some
resources in applications that only do progressive reading.

This reverts commit f4276c36.

having to do with sCAL chunk handling

sig_bit was not declared in write_png() and was used differently from
sig_bit in read_png(), as reported by Vincent Torri.  This update changes
sig_bit to sig_bit_p in read_png() and declares sig_bit in write_png().

The memset() should use "row_bytes" instead of "png_ptr->rowbytes".
This bug was introduced in libpng-1.2.36.

The variable "gamma" is also defined in math.h on some platforms.