[libpng17] Added libpng 1.5 checks to pngvalid.c
[libpng17] Merged with 1.5 and 1.6 changes to create a single pngvalid.c
[libpng17] Added dSIG to chunk list in png.h
[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
[libpng17] 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.