[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.
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>