preprocessor/6521: -MG creates dependency with wrong path

[Neil Booth]

Zack Weinberg wrote:-

> This is true.  And we have one registered complaint about the current
> behavior.  We can always put it back later.
> 
> Robert, would you be willing to clarify the documentation?

OK, I'll apply Robert's patch.  What do you feel is unclear about
the docs?  The fact that it refers to "current directory"?

Neil.