make dependency on parts of a file

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

Suppose I have a file A and another file B depending on only parts of A.
That is, if (the time stamp of) A changes, it is quite likely that B 
(derived from A) will not.
Suppose it's cheap to build B from A (or to check whether B actually changed).
Then suppose there's C, which depends on B but is expensive to build.

Is there a sane way to rebuild C only if the relevant parts of A changed?

If you prefer actual examples, A is my network configuration file, 
B is the list of Access Points and C is the RADIUS client configuration.
The thing I'm trying to avoid is restarting the RADIUS server each time 
something changed in the network config.