only works with libpng versions that can be made to reliably fail when
the deflate data contains an out-of-window reference. This means only
1.6 and later.
Fixed gnu issues: g++ needs a static_cast, gcc 4.4.7 has a broken warning
message which it is easier to work round than ignore.
For inflate, reverted previous fixes.
Added fixitxt and pngdeflate to the built programs and removed warnings
from the source code and timepng that are revealed as a result. Fixed
fixitxt when the chunk length is more than 65535 (untested, no test case).
