Don't attempt to intercept syscall(2) if not available.

Index: src/lib/torsocks.c
--- src/lib/torsocks.c.orig
+++ src/lib/torsocks.c
@@ -240,11 +240,15 @@ static void init_libc_symbols(void)
 	tsocks_libc_connect = dlsym(libc_ptr, LIBC_CONNECT_NAME_STR);
 	tsocks_libc_close = dlsym(libc_ptr, LIBC_CLOSE_NAME_STR);
 	tsocks_libc_socket = dlsym(libc_ptr, LIBC_SOCKET_NAME_STR);
+#ifdef HAVE_SYSCALL
 	tsocks_libc_syscall = dlsym(libc_ptr, LIBC_SYSCALL_NAME_STR);
+#endif
 	tsocks_libc_execve = dlsym(libc_ptr, LIBC_EXECVE_NAME_STR);
-	tsocks_libc_accept4 = dlsym(libc_ptr, LIBC_ACCEPT4_NAME_STR);
 	if (!tsocks_libc_connect || !tsocks_libc_close || !tsocks_libc_socket ||
-			!tsocks_libc_syscall || !tsocks_libc_execve || ! tsocks_libc_accept4) {
+#ifdef HAVE_SYSCALL
+			!tsocks_libc_syscall ||
+#endif
+			!tsocks_libc_execve) {
 		ERR("Unable to lookup symbols in " LIBC_NAME "(%s)", dlerror());
 		goto error;
 	}
