BUILD: signals: FreeBSD has SI_LWP instead of SI_TKILL

SI_TKILL is for Linux. We're again in the non-portable area. Both OSes
use macros to define these values so we can #ifdef them. Let's make
SI_TKILL defined based on SI_LWP when only the latter is defined.

include/common/compat.h

@@ -23,6 +23,7 @@
 #define _COMMON_COMPAT_H
 
 #include <limits.h>
+#include <signal.h>
 #include <time.h>
 #include <unistd.h>
 /* This is needed on Linux for Netfilter includes */
@@ -104,6 +105,11 @@ typedef struct { } empty_t;
 #define F_SETPIPE_SZ (1024 + 7)
 #endif
 
+/* On FreeBSD we don't have SI_TKILL but SI_LWP instead */
+#if !defined(SI_TKILL) && defined(SI_LWP)
+#define SI_TKILL SI_LWP
+#endif
+
 /* systems without such defines do not know clockid_t or timer_t */
 #if !(_POSIX_TIMERS > 0)
 #undef clockid_t