Modify <sysname>.make, not system.make or sysHACK.make.

system.make is usually just a generated copy of <sysname.make>.
sysHACK.make is usually just a generated copy of <sysname.make>
except on Sun Workstations using /bin/cc, then it is a generated
copy of sunHACK.make.

The generated copy (system.make) is generated by the OTSO root
Makefile: e.g. 	make config-sun-owc
The generated copy (system.make) will have ENVFLAGS = -D__OWCCPP__
in this case.  sun.make does not.
