Hierarchical automatic variables?

[Peart, Jonathan E]

According to the gmake manual, the automatic variable $? gives you "The names 
of all the prerequisites that are newer than the target, with spaces between 
them."
That only works on one level of hierarchy.  What if I wanted the names of all 
prerequisites of the prerequisites of the prerequisites, etc etc.
A : B
B : C
C : D
For example: all targets need to be remade and I want all the prerequisites of 
target A which should be B, C, and D.  Whereas $? works on one level and would 
return B as the only prerequisite of target A.
Thank you in advancd for any help,


Jonathan Peart