perf trace: Warn the user when not available

When the audit-libs devel package is not found at build time we disable
the 'trace' command, as we are not able to map syscall numbers to
strings, but then the message the user is presented is cryptic:

  [root@zoo linux]# trace ls
  perf: 'ls' is not a perf-command. See 'perf --help'.

Fix it by presenting a more helpful message:

  [root@zoo linux]# trace l
  trace command not available: missing audit-libs devel package at build time.

Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Don Zickus <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Jiri Olsa <[email protected]>

tools/perf/perf.c

@@ -481,14 +481,18 @@ int main(int argc, const char **argv)
 		fprintf(stderr, "cannot handle %s internally", cmd);
 		goto out;
 	}
-#ifdef HAVE_LIBAUDIT_SUPPORT
 	if (!prefixcmp(cmd, "trace")) {
+#ifdef HAVE_LIBAUDIT_SUPPORT
 		set_buildid_dir();
 		setup_path();
 		argv[0] = "trace";
 		return cmd_trace(argc, argv, NULL);
-	}
+#else
+		fprintf(stderr,
+			"trace command not available: missing audit-libs devel package at build time.\n");
+		goto out;
 #endif
+	}
 	/* Look for flags.. */
 	argv++;
 	argc--;