finding datadir from executable

[Nicholas R. Markham]

I recently decided to try to apply the suggestion mentioned in section 17.5 of
the autoconf manual - namely, to hardcode only the relative path from prefix to
datadir (or maybe from bindir to datadir) and try to determine prefix at
runtime.  However, I'm not sure how to determine prefix at runtime.  Can anyone
give me an example of how they did it?  Thanks.