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

Index: src/lib/syscall.c
--- src/lib/syscall.c.orig
+++ src/lib/syscall.c
@@ -483,6 +483,7 @@ static LIBC_SYSCALL_RET_TYPE handle_passthrough(long n
 /*
  * Torsocks call for syscall(2)
  */
+#ifdef HAVE_SYSCALL
 LIBC_SYSCALL_RET_TYPE tsocks_syscall(long int number, va_list args)
 {
 	LIBC_SYSCALL_RET_TYPE ret;
@@ -636,7 +637,9 @@ LIBC_SYSCALL_DECL
 
 	return ret;
 }
+#endif /* HAVE_SYSCALL */
 
+#ifdef HAVE___SYSCALL
 /* Only used for *BSD systems. */
 #if (defined(__NetBSD__) || defined(__FreeBSD__))
 
@@ -703,3 +706,4 @@ LIBC___SYSCALL_DECL
 }
 
 #endif /* __NetBSD__, __FreeBSD__ */
+#endif /* HAVE___SYSCALL */
