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>

by earlier versions of GCC fixed for Cygwin and Min/GW (which both use old GCCs.)
ARM support is enabled by default in zlib.props (unsupported by Microsoft) and
ARM compilation is made possible by deleting the check for x86. The test programs
cannot be run because they are not signed.

Also added pngunkown to the vstudio builds, fixed build errors and corrected
a minor exit code error in pngvalid if the 'touch' file name is invalid.

The "_DLL" is removed from the test of whether a DLL is being built.

Also corrected zlib.vcxproj into which Visual Studio had introduced
what it calls an "authoring error".  The change to make pnglibconf.h
simply copies the file; in the future it may actually generate the
file from scripts/pnglibconf.dfa as the other build systems do.

  The changes include support for function attributes in VC in addition to
    those already present in GCC - necessary because without these some
    warnings are unavoidable.  Fixes include signed/unsigned fixes in
    pngvalid and checks with gcc -Wall -Wextra -Wunused.
  VC requires function attributes on function definitions as well as
    declarations, PNG_FUNCTION has been added to enable this and the
    relevant function definitions changed.

that were recently introduced, mostly resulting from
safe but uncast assignments to shorter integers.  Also added a zlib
VStudio release library project because the latest zlib Official Windows
build does not include such a thing.

which will be removed from libpng-1.5.0.