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

ported from libpng-1.4.0

(inside an #if 0/#endif block)

and avoid reference to time.h "tm" structure in png.h and pngwrite.c

    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.

    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.

by defining PNG_iTXT_SUPPORTED

a 16-bit int (assuming height immediately follows width).