Solaris needs -lresolv, ie $(LIBRESOLV), for anything referencing DNS.
I added _res stuff to os.c.
os.c is used by multiple tools, but of those only Exim needs the DNS functionality.
So used a !defined(COMPILE_UTILITY) guard.
Other OSs handle thread-safe resolver differently, in ways which fail if the
programmer creates their own structs. */
-#ifndef OS_GET_DNS_RESOLVER_RES
+#if !defined(OS_GET_DNS_RESOLVER_RES) && !defined(COMPILE_UTILITY)
#include <resolv.h>