What is the "most standard" c++ md5 header?

I want to determine if files have been modified by the user so that my program will leave them unmodified when doing updates. Like YaST does. I plan on using MD5 to generate a hash to check when visiting files. I have sourcecode that claims to be genuine RSA MD5, so getting something that works isn't a problem. I would, however, prefer to build against a "standard" library rather than something I add to my own source tree just to get things working. Is there a good candidate for providing MD5 hashing that will be available on most systems? Steven