Open the Terminal app (located in the Utilities folder of Applications) and enter the following command:
defaults write com.apple.LaunchServices LSQuarantine -bool NO
This command turns off the hidden Mac OS X setting for these warnings. Once you're done, you need to restart Finder. You can do this by logging off. You can also restart Finder by using the "Force Quit Applications" feature in Mac OS X, which can be opened with the keyboard shortcut Command+Option+Esc (known as Alt+Windows+Esc on Windows keyboards).
To turn the warnings back on, just enter the same line of code in Terminal, but replace "NO" with "YES".
If you only want to turn off internet download warnings for a specific folder, use this command instead:
xattr -d -r com.apple.quarantine FolderPath
Replace "FolderPath" with the location of the folder where you want to turn off the warnings. For example, "/System/Library/Extensions" (without the quotation marks).
Hopefully, with this quick and easy tip, you can make your Hackintosh's file system is happier and slightly less annoying place.