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.

fixed Cygwin+MSVC build errors. The ICC profile handling now includes more
checking.  Several errors that caused rejection of the profile are now
handled with a warning in such a way that the invalid profiles will be
read by default in release (but not pre-RC) builds but will not be
written by default.  The easy part of handling a cHRM chunk is written,
where the ICC profile contains the required data.  The more difficult
part plus guessing a gAMA value requires code to pass selected RGB values
through the profile.