Detect whether or not the deprecated AuthorizationExecuteWithPrivileges function exists before using

Pattern by Andy Kim from Potion Factory LLC wrote some code with the following pattern and comments. This also helps prevent Xcode from declaring the deprecated function as being such. 

On 10.7+, AuthorizationExecuteWithPrivileges is deprecated. We want to still use it since there's no good alternative (without requiring code signing). We'll look up the function through dyld and fail if it is no longer accessible. If Apple removes the function entirely this will fail gracefully. If they keep the function and throw some sort of exception, this won't fail gracefully, but that's a risk we'll have to take for now.