add NOPNG to cmake to explicitly disable PNG dependency
reorganise osx detection and variable initalization (simpler crosscompiling) in cmake
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>get-task-allow</key>
<true/>
</dict>
</plist>