add an option to use sig{set,long}jmp and default it to on for linux systems

generic.h

@@ -284,6 +284,7 @@ There's a BIG swamp lurking where network code of any sort lives.
 #undef HAVE_SYSINFO
 #undef HAVE_SELECT_H
 #undef HAVE_TTYENT_H
+#define HAVE_SIGSETJMP
 #define HAVE_SOCKLEN_T
 #endif /* linux */
 

netcat.c

@@ -102,6 +102,14 @@
 #define socklen_t int
 #endif
 
+#ifdef HAVE_SIGSETJMP
+#define jmp_buf sigjmp_buf
+#undef setjmp
+#define setjmp(buf) sigsetjmp(buf,1)
+#undef longjmp
+#define longjmp siglongjmp
+#endif
+
 struct host_poop {
   char name[MAXHOSTNAMELEN];	/* dns name */
   char addrs[8][24];		/* ascii-format IP addresses */