Fixes trac:2768: exclude opal/libltdl from "make distclean" when

--disable-dlopen is used.  Thanks to David Gunter for reporting the
issue. 

This commit was SVN r24603.

The following Trac tickets were found above:
  Ticket 2768 --> https://svn.open-mpi.org/trac/ompi/ticket/2768

opal/Makefile.am

@@ -9,7 +9,7 @@
 #                         University of Stuttgart.  All rights reserved.
 # Copyright (c) 2004-2005 The Regents of the University of California.
 #                         All rights reserved.
-# Copyright (c) 2009-2010 Cisco Systems, Inc.  All rights reserved.
+# Copyright (c) 2009-2011 Cisco Systems, Inc.  All rights reserved.
 # $COPYRIGHT$
 # 
 # Additional copyrights may follow
@@ -32,9 +32,12 @@ SUBDIRS = \
         . \
 	$(MCA_opal_FRAMEWORK_COMPONENT_DSO_SUBDIRS)
 
+# libltdl is included by variable because if --disable-dlopen was
+# used, there will be no generated Makefile in that directory (and
+# therefore make distclean will fail).
 DIST_SUBDIRS = \
 	include \
-	libltdl \
+	$(LIBLTDL_SUBDIR) \
 	asm \
         datatype \
         etc \