Some platforms don't have <execinfo.h>, even if they have backtrace()

function (e.g., NetBSD).  Thanks to Aleksej Saushev for pointing out
the issue. 

This commit was SVN r24866.

opal/mca/backtrace/execinfo/backtrace_execinfo.c

@@ -9,6 +9,7 @@
  *                         University of Stuttgart.  All rights reserved.
  * Copyright (c) 2004-2006 The Regents of the University of California.
  *                         All rights reserved.
+ * Copyright (c) 2011 Cisco Systems, Inc.  All rights reserved.
  * $COPYRIGHT$
  * 
  * Additional copyrights may follow
@@ -19,7 +20,9 @@
 #include "opal_config.h"
 
 #include <stdio.h>
+#ifdef HAVE_EXECINFO_H
 #include <execinfo.h>
+#endif
 
 #include "opal/constants.h"
 #include "opal/mca/backtrace/backtrace.h"