Exim fails to build, complaining about the absence of the killpg function.

This function should be present in all modern flavours of Unix. If you are using an older version, you should be able to get round the problem by inserting

#define killpg(pgid,sig)   kill(-(pgid),sig)

into the file called OS/os.h-xxx, where xxx identifies your operating system, and is the output of the command scripts/os-type -generic.