Debugging Lillypond - Unable to find file "lily.scm" in load path

[Tom]

I have a working version of lilypond and am trying to build a debug 
version to see how it works to see if I can help but when I try and 
debug it cant initialise:
 Unable to find file "lily.scm" in load path
I could, presumably copy the working versions /usr/share/lilypond/2.16.0 
directory structue to build/lily/share but it may be out of sync?
I'd rather not do an install and then recompile the current version with 
debugging data so how do I get the share data built and installed?
Tom