bsd_signal is a legacy (removed) XSI function, not needed in _BSD_SOURCE

its only purpose was for use on non-BSD systems that implement sysv
semantics for signal() by default.

include/signal.h

@@ -205,16 +205,13 @@ void (*sigset(int, void (*)(int)))(int);
 #define SIGSTKSZ 8192
 #endif
 
-#if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
-void (*bsd_signal(int, void (*)(int)))(int);
-#endif
-
 #ifdef _BSD_SOURCE
 typedef void (*sig_t)(int);
 #endif
 
 #ifdef _GNU_SOURCE
 typedef void (*sighandler_t)(int);
+void (*bsd_signal(int, void (*)(int)))(int);
 int sigisemptyset(const sigset_t *);
 #define SA_NOMASK SA_NODEFER
 #define SA_ONESHOT SA_RESETHAND