[libpng15] Imported from libpng-1.5.18beta02.tar
[libpng15] Bump version to 1.5.18beta03
[libpng15] Fix typo in CHANGES and ANNOUNCE
[libpng15] Removed configure-generated "compile" file.
[libpng15] Fix typo in pngrtran.c
[libpng15] Re-added "compile" (libpng15 branch includes configure-generated

stuff).
[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