Ensure the hwloc external header is include when --with-devel-headers is given

opal/mca/hwloc/external/Makefile.am

@@ -1,5 +1,6 @@
 #
 # Copyright (c) 2011-2012 Cisco Systems, Inc.  All rights reserved.
+# Copyright (c) 2016      Intel, Inc. All rights reserved.
 # $COPYRIGHT$
 #
 # Additional copyrights may follow
@@ -14,6 +15,7 @@
 # We only ever build this component statically
 
 noinst_LTLIBRARIES = libmca_hwloc_external.la
+
 libmca_hwloc_external_la_SOURCES = \
         external.h \
         hwloc_external_component.c
@@ -21,3 +23,9 @@ libmca_hwloc_external_la_LDFLAGS = \
     -module -avoid-version \
     $(opal_hwloc_external_LDFLAGS)
 libmca_hwloc_external_la_LIBADD = $(hwloc_external_LIBS) -lhwloc
+
+# Conditionally install the header files
+if WANT_INSTALL_HEADERS
+opaldir = $(opalincludedir)/$(subdir)
+nobase_opal_HEADERS = external.h
+endif