Agreed. I have never seen $DESTDIR prefixed with / in makefiles, so this patch should go in.

I don't see the point of making --prefix relative. 
you can do 

 --prefix=`cd ../relative ; pwd` 

if you need relative.

Putting relative paths in makefiles is asking for trouble, as the Makefile will call other makefiles in subdirectories, which would need to pass extra ../ overrides of prefix.