[libpng15] Fixed 'unused' warnings and mergeed with libpng16 pngvalid.

Various warnings revealed by configurations added to the tested
configurations in libpng 1.6 need corresponding fixes in libpng 1.5.
This takes the libpng 1.6 pngvalid.c and adds it back to libpng 1.5,
however one write-only test is currently disabled because it does not
work; possibly because of failures to reset or initialize the deflate
stream correctly in write-only cases.
[libpng15] Merged libpng-1.7 changes and created a single pngvalid.c
[libpng15] Merged pngvalid and pngtest from libpng-1.5; made compressed chunks

after IDAT work with the progressive read code (this is a minor and
somewhat ugly fix-up; the proper fix is in 1.6).
[libpng15] The 'make all-am' command no longer builds $(check_PROGRAMS), leading

to a lack of checks on cross-builds such as ARM.  Makefile.am also
requires correct dependencies for the check programs on pnglibconf.h
otherwise parallel builds on fast machines can sometimes pick up the
system pnglibconf.h.
[libpng15] Rebuilt configure scripts with automake 1.14.1 and autoconf-2.69.

Require automake-1.13 and enable automake "subdirs" feature in autoconf.ac.
[libpng15] Revised a dependency in Makefile.am to work with 'subdirs' option.
[libpng15] Pushed new pngvalid.c to the libpng15 branch.
[libpng15] Removed extraneous "#endif" from ANNOUNCE.
[libpng15] 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.