Add -w/--where to show backtraces of traced functions.

- add libunwind as a dep to configure
- add a command line option to show backtraces from traced library functions
- best guess at autoconf stuff, seems to work x86_64 and i686
- NOT TESTED ON ANY OTHER PLATFORMS!
diff --git a/options.h b/options.h
index e2d0e7f..d171c4b 100644
--- a/options.h
+++ b/options.h
@@ -16,6 +16,9 @@
 	int follow;     /* trace child processes */
 	int no_plt;     /* set bps on PLT entries */
 	int no_signals; /* don't print signals */
+#if defined(HAVE_LIBUNWIND)
+	int bt_depth;	 /* how may levels of stack frames to show */
+#endif /* defined(HAVE_LIBUNWIND) */
 };
 extern struct options_t options;