to catch library and header mismatch, also found at the end of pngtest.c.

that CCP writes to standard output by default (Martin Banky).

Studio 2010 (John Bowler).

and shortening problems on assignment but also a few difficult to optimize
(for GCC) loops (John Bowler).

literal pointers declare them (John Bowler).
Many APIs did not change their arguments but were not declared using
PNG_CONST.  This change corrects this.  In a few cases APIs that return
constant string literal pointers have also been changed to declare this.
Unlike the argument change this may require app changes; however the
results could never be written to (the app would crash on some platforms
where strings are not writable), so this seems advisable.

  The changes include support for function attributes in VC in addition to
    those already present in GCC - necessary because without these some
    warnings are unavoidable.  Fixes include signed/unsigned fixes in
    pngvalid and checks with gcc -Wall -Wextra -Wunused.
  VC requires function attributes on function definitions as well as
    declarations, PNG_FUNCTION has been added to enable this and the
    relevant function definitions changed.

Also png_set_filter_heuristics_fixed and png_get_aspect_ratio_fixed.