Only #define PNG_DEPSTRUCT, etc. in pngconf.h if not already defined.

in pngtest.c that were detected by the new PNG_DEPSTRUCT macro.

ported from libpng-1.4.0

as an aid to migration to 1.4.x

to support WindowsCE.

Revised iTXt support. To ensure binary compatibility with
previous versions, the "lang" and "lang_key" members will be assumed
to be omitted from versions prior to 1.2.41beta11 whenever there is a
library mismatch.

    To ensure binary compatibility with
    previous versions, the "lang" and "lang_key" members will be assumed
    to be omitted from previous versions unless the current libpng
    version was built with PNG_iTXt_SUPPORTED (which is otherwise no
    longer necessary to gain iTXt support), as a signal that the user has
    been building previous versions with PNG_iTXt_SUPPORTED as well.

Merged libpng-1.2.41.txt with libpng-1.4.0.txt where appropriate.

into the main libpng directory.  More bugfixes and improvements to
CMakeLists.txt (Philip Lowman)

from pngwrite.c

    png_calloc(), png_get_io_state(),
    png_get_io_chunk_name(), png_set_premultiply_alpha, and
    png_do_read_premultiply_alpha().

changing "#if defined(x)" to "#ifdef x" in png.h and pngconf.h.

everywhere except pngconf.h as in libpng-1.4.0beta78 and later.