After compiling Squid, remove one or more of the .o files and run make again.
Use the ulimit or limits command to change the file descriptor limit to some small value before compiling Squid. Does ./configure obey or ignore your new limit?
Compile Squid with a high file-descriptor limit, then try to run it on a system with a lower limit. Does Squid use the lower or higher limit?
What happens if you mistype one of the enable options? What if you specify an invalid storage scheme with the enable-store-io option?
After compiling Squid, remove src/Makefile and try to compile it again. What's the easiest way to restore the file?