Fixes:
agent.c: In function '_DtCm_init_agent':
agent.c:160: warning: passing argument 5 of 'registerrpc' from incompatible pointer type
agent.c:160: warning: passing argument 6 of 'registerrpc' from incompatible pointer type
agent.c:167: warning: passing argument 5 of 'registerrpc' from incompatible pointer type
agent.c:167: warning: passing argument 6 of 'registerrpc' from incompatible pointer type
cpp was removed from /usr/libexec with FreeBSD 5.0-RELEASE.
Use __FreeBSD_version to tell imake if it's still there.
While here, X.Y.Z versioning ended with FreeBSD 3.0, so
we must be dealing with 2.Y.Z when checking for -lgnumalloc.