to enable the SSE optimization

Configure libpng with "configure --enable-intel-see" or compile
libpng with "-DPNG_INTEL_SSE" in CPPFLAGS to enable it.

accomodate up to 16 options.

Added option to Cmake build allowing a custom location of Zlib to be specified. This is useful in a scenario where libpng is being built as a subproject alongside zlib by another project..

(bug report and patch by Patrick Keshishian).

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.