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 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.
