core: always compile the raw sockets code on linux

Raw sockets support is compiled always on linux.
Note that while the code is there, one still needs to explicitly
enable them in ser.cfg or at runtime (e.g. setting  udp4_raw = 1 at
runtime will switch to using raw sockets for sending if possible).

Makefile.defs

@@ -1628,7 +1628,7 @@ ifeq ($(OS), linux)
 	use_futex= yes
 	C_DEFS+=-DHAVE_GETHOSTBYNAME2 -DHAVE_UNION_SEMUN -DHAVE_SCHED_YIELD \
 			-DHAVE_MSG_NOSIGNAL -DHAVE_MSGHDR_MSG_CONTROL -DHAVE_ALLOCA_H \
-			-DHAVE_TIMEGM -DHAVE_SCHED_SETSCHEDULER
+			-DHAVE_TIMEGM -DHAVE_SCHED_SETSCHEDULER -DUSE_RAW_SOCKS
 	ifneq ($(found_lock_method), yes)
 		#C_DEFS+= -DUSE_POSIX_SEM
 		C_DEFS+=-DUSE_PTHREAD_MUTEX