Do not print anything when self-backtrace unavailable

Right now, gdb's self-backtrace feature will still print something
when a backtrace is unavailable:

   sig_write (_("----- Backtrace -----\n"));
[...]
     sig_write (_("Backtrace unavailable\n"));
    sig_write ("---------------------\n");

However, if GDB_PRINT_INTERNAL_BACKTRACE is undefined, it seems better
to me to print nothing at all.

This patch implements this change.  It also makes a couple of other
small changes in this same module: it adds a header guard to
bt-utils.h, and it protects the definitions of
gdb_internal_backtrace_1 with a check of GDB_PRINT_INTERNAL_BACKTRACE.

gdb/bt-utils.h

@@ -18,6 +18,9 @@
 /* Support for printing a backtrace when GDB hits an error.  This is not
    for printing backtraces of the inferior, but backtraces of GDB itself.  */
 
+#ifndef BT_UTILS_H
+#define BT_UTILS_H
+
 #ifdef HAVE_LIBBACKTRACE
 # include "backtrace.h"
 # include "backtrace-supported.h"
@@ -67,3 +70,5 @@ extern void gdb_internal_backtrace ();
 
 extern void gdb_internal_backtrace_set_cmd (const char *args, int from_tty,
 					    cmd_list_element *c);
+
+#endif /* BT_UTILS_H */