[libpng17] Removed #error macro from contrib/tools/pngfix.c (Thomas Klausner).
[libpng16] Merged pngrio.c, pngtrans.c, and pngwio.c with libpng-1.6.9
[libpng17] Fixed test programs for interlace options. Made pngvalid.c and

pngtest.c work correctly when READ_INTERLACING and/or WRITE_INTERLACING
are switched off.
[libpng17] Changed pngvalid.c to support libpng 1.5, which does not support

the PNG_MAXIMUM_INFLATE_WINDOW option, so #define it out when
appropriate in pngvalid.c
Allow unversioned links created on install to be disabled in configure
In configure builds 'make install' changes/adds links like png.h and libpng.a to
point to the newly installed, versioned, files (e.g. libpng17/png.h and
libpng17.a).  Three new configure options and some rearrangement of Makefile.am
allow creation of these links to be disabled.

Signed-off-by: John Bowler <jbowler@acm.org>