How to debug linker errors?

[walt]

walt wrote:

The problems I'm talking about are 'missing symbol' errors
occuring at link time, not at compile time.
Well, I answered my own question by looking at the patch
that was posted for the FBSD kernel this evening -- perfect
timing.
I see that one of the object files being linked must define
the symbol for all the other object files which need it --
simple and logical.
But I'm still not sure how to deduce which of the object files
*should* be defining the missing symbol.  Or (as in today's
FBSD kernel bug) maybe the missing symbol is defined in a source
file which didn't get compiled at all, depending on the kernel
configuration.
So, how to track down which object file should define the symbol?
Is there a logical and methodical approach to the problem?
Any hints much appreciated!