contrib/pngminim/decoder/pngusr.h to make a smaller decoder application.

For Windows/x86 platforms only:
  __stdcall is no longer needed for Visual Basic, so libpng-1.5.0 uses
  __cdecl throughout (both API functions and callbacks) on Windows/x86
  platforms.

This allows the functions
to read and write ints to be disabled independently of PNG_USE_READ_MACROS,
which allows libpng to be built with the functions even though the default
is to use the macros - this allows applications to choose at app build
time whether or not to use macros (previously impossible because the
functions weren't in the default build.)

It's now already on by default.

save some space.

    contrib/pngminim project to pngerror.c, for use when warnings or
    errors are disabled via PNG_NO_WARN or PNG_NO_ERROR_TEXT, to avoid
    storing unneeded error/warning text.

not quite working yet (pow is undefined)

and fixed a small typo in a comment in pngconf.h

version that works with libpng-1.5.0 is received)

    In all prior versions of libpng most configuration options
    controlled by compiler #defines had to be repeated by the
    application code that used libpng.  This patch changes this
    so that compilation options that can only be changed at build
    time are frozen in the build.  Options that are compiler
    dependent (and those that are system dependent) are evaluated
    each time - pngconf.h holds these.  Options that can be changed
    per-file in the application are in png.h.  Frozen options are
    in the new installed header file pnglconf.h