ompi/include/Makefile.am: rm mpi_portable_platform.h first

We've seen this a few times (e.g.,
http://www.open-mpi.org/community/lists/users/2015/06/27057.php
reported via @siegmargross).  I'm not entirely sure why it happens --
the best I can come up with is a poorly-synchronized network
filesystem and/or a bug in "make".  For example: this code hasn't
changed in forever, and it only happens to users *sometimes*.

Regardless, avoid the error altogether by removing the file before
making the sym link (it should be a sym link anyway -- if there's
something there, it should be safe to remove it before we re-create
the sym link that should be there in the first place).

(cherry picked from commit 0edd265ea045e649c9489e3cb8fdb657800d95c3)

ompi/include/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) 2006-2014 Cisco Systems, Inc.  All rights reserved.
+# Copyright (c) 2006-2015 Cisco Systems, Inc.  All rights reserved.
 # Copyright (c) 2009-2011 Oak Ridge National Labs.  All rights reserved.
 # Copyright (c) 2014-2015 Research Organization for Information Science
 #                         and Technology (RIST). All rights reserved.
@@ -106,4 +106,5 @@ distclean-local:
 	rm -f mpi-ext.h mpif-ext.h mpi_portable_platform.h mpif-sizeof.h
 
 mpi_portable_platform.h: $(top_srcdir)/opal/include/opal/opal_portable_platform.h
+	-@rm -f mpi_portable_platform.h
 	$(OMPI_V_LN_S) $(LN_S) $(top_srcdir)/opal/include/opal/opal_portable_platform.h mpi_portable_platform.h