If you are compiling sendmail on a SysVR4-derived machine, you should define SYSTEM5. This automatically causes the correct SysV support to be included. For all systems that require SYSTEM5 to be defined, it is already correctly defined in sendmail/conf.h (include/sm/conf.h beginning with V8.12).

If you suspect that you need to define SYSTEM5 when porting to a new system, you should also investigate SYS5SIGNALS and SYS5SETPGRP in sendmail/conf.h (include/sm/conf.h beginning with V8.12) and sendmail/README. If you are running a precompiled version of sendmail, you can use the -d0.10 debugging command-line switch (-d0.10) to discover whether SYSTEM5 or SYS5SETPGRP is defined (if either appears in the list, it is defined).