This removes the use of a macro containing the pre-processor 'defined'
operator.  It is unclear whether this is valid; a macro which
"generates" 'defined' is not permitted, but the use of the work
"generates" within the C90 standard seems to imply more than simple
substitution of an expression itself containing a well-formed defined
operation.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

chunk handling.

avoid ADLER32 evaluation.

to avoid a signed/unsigned compare in the preprocessor.

Fixed offsets in contrib/intel/intel_sse.patch

tarballs.

This reverts commit 97dfccb6.

pngwrite.c, pngunknown.c, and pngvalid.c.  Several warnings still remain
in pngvalid.c

Remove all currently detected cases of unsigned overflow.  Detection is
runtime, so test case dependent.  The changes to pngvalid.c eliminate
spurious and probably invalid tests with one while loop exception.

Apart from that and the change to the dependence on the intended
unsigned overflow in pngtrans.c the changes are limited to altering the
meme for an unsigned 'x' from:

   while (x-- > 0)

to

   for (; x > 0; --x)

This works because, in all cases, the control variable is not used in
the loop.  The 'while' meme was, at one time, warn'ed by GCC so it is
probably a good change, for some weird religious value of good.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

flag definitions in pngpriv.h from 0xnnnn to 0xnnnnU and trivial changes
in png.c, pngread.c, and pngwutil.c.