Let's avoid using md5 as checksum

[Jordan K. Hubbard]

Given all the other unfinished or unstarted work in MacPorts which  
needs to happen just to get the collection halfway reliable, it seems  
to me that arguing over the safety of a commonly used checksum is  
little more than a distraction and represents time that could be  
devoted to more important pursuits, like cleaning up the variant code,  
or getting a distributed unit testing setup going, or ...

- Jordan