hwloc/external.h: fix a clash with external HWLOC_VERSION[*]

Some macros defined by the embedded hwloc ends up in opal_config.h
because hwloc configury m4 files are slurped into Open MPI.  These
macros are not required here, and they might conflict with an external
hwloc install, so simply #undef them in hwloc/external/external.h
after including <opal_config.h> but before including the external
<hwloc.h>.

Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

opal/mca/hwloc/external/external.h

@@ -1,6 +1,6 @@
 /*
- * Copyright (c) 2011-2017 Cisco Systems, Inc.  All rights reserved
- * Copyright (c) 2016      Research Organization for Information Science
+ * Copyright (c) 2011-2019 Cisco Systems, Inc.  All rights reserved
+ * Copyright (c) 2016-2019 Research Organization for Information Science
  *                         and Technology (RIST). All rights reserved.
  *
  * Copyright (c) 2016-2017 Intel, Inc. All rights reserved.
@@ -21,6 +21,23 @@
 BEGIN_C_DECLS
 
 #include <opal_config.h>
+
+/* Top-level configure will always configure the embedded hwloc
+ * component, even if we already know that we'll be using an external
+ * hwloc (because of complicated reasons). A side-effect of this is
+ * that the embedded hwloc will AC_DEFINE HWLOC_VERSION (and friends)
+ * in opal_config.h. If the external hwloc defines a different value
+ * of HWLOC_VERSION (etc.), we'll get zillions of warnings about the
+ * two HWLOC_VERSION values not matching.  Hence, we undefined all of
+ * them here (so that the external <hwloc.h> can define them to
+ * whatever it wants). */
+
+#undef HWLOC_VERSION
+#undef HWLOC_VERSION_MAJOR
+#undef HWLOC_VERSION_MINOR
+#undef HWLOC_VERSION_RELEASE
+#undef HWLOC_VERSION_GREEK
+
 #include MCA_hwloc_external_header
 
 /* If the including file requested it, also include the hwloc verbs