The commit removes the three compile warning options from the individual project
files into the zlib.props globals (this should probably be renamed in 1.7).  It
increases the warning level from 4 to All and adds a list of the warnings which
need to be turned off.  This is semi-documentary; the intent is to tell users of
libpng which warnings have been examined and judged non-fixable at present.

The warning about structure padding *IS* fixable, but it is a signficant change
(moving structure members around).  It is currently partially fixed in 1.7, but
only partially.
Signed-off-by: NJohn Bowler <jbowler@acm.org>