Note: the new 3.30 beta release of httrack currently 
available (beta-4) is now able to load external modules 
directly without recompiling.

Compile the given example (callbacks-example.c) as a module 
(callback.so or callback.dll), such as
      gcc -O -g3 -Wall -D_REENTRANT -DINET6 -
D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -shared -o 
callback.so callbacks-example.c

And then, use the new --wrapper option in httrack:
      httrack --wrapper check-html=callback:process_file
              --wrapper link-
detected=callback:check_detectedlink
              --wrapper loop=callback:check_loop
              ...

Your external wrappers will then be called for the three 
callbacks defined.