hwloc201/configure.m4: make it safe when used with hwloc:external

The Autoconf AC_CONFIG_* macros can only be instantiated exacly once
for any given file, *and* they must be in a code execution path at run
time for the target file to be generated at the end of configure.

For example, if you want to generate file ABC at the end of configure,
you must invoke the AC_CONFIG_FILES(ABC) macro in a code path that
will get executed when configure is run.

That's pretty straightforward.

What's not straightforward is two corner cases:

1. You cannot invoke the AC_CONFIG_FILES(ABC) macro for the same file
   more than once.  If you do, autoreconf will fail (even before you
   can run configure).
2. If AC_CONFIG_FILES(ABC) is not in a code path that is executed by
   configure, the file ABC is not registered properly, and ABC will
   not be generated at the end of configure.

This applies to hwloc because hwloc's HWLOC_SETUP_CORE macro calls
both AC_CONFIG_FILES and AC_CONFIG_HEADER to setup its Makefiles
(etc.) so that targets like "make distclean" and "make distcheck" will
work properly.  Hence, we *have* to invoke HWLOC_SETUP_CORE.

However, the MCA_opal_hwloc_hwloc201_CONFIG macro has a few side
effects.  It would be nice to do able to do something like this:

```
    if hwloc:extern is going to be used:
        Invoke minimal HWLOC_SETUP_CORE (with no side effects)
    else
        Invoke full HWLOC_SETUP_CORE (with side effects)
    fi
```

But we can't, because autoreconf will detect that AC_CONFIG_FILES has
been invoked on the same files more than once (regardless of whether
those code paths will be executed at run time or not).  Kaboom.

Similarly, we can't do this:

```
    if hwloc:extern is not going to be used:
        Invoke full HWLOC_SETUP_CORE (with side effects)
    fi
```

Because then hwloc's AC_CONFIG_FILES won't be registered properly when
hwloc:external *is* used (i.e., when the HWLOC_SETUP_CORE macro is not
in a code path that is executed at run time), and targets like "make
distclean" will fail because hwloc's Makefiles won't have been setup.
Kaboom.

But remember that the hwloc framework is a bit special: there will
only ever be 2 comoponents: external and internal.  External is
guaranteed to be configured first because of its priority.  So the
internal component (i.e., this component) immediately knows if it is
going to be used or not based on whether the external component
configuration succeeded or failed.

Specifically: regardless of whether the internal component (i.e., this
component) is going to be used, we have to invoke HWLOC_SETUP_CORE.
But we can manage the side effects: allow the side effects when
this/internal component is going to be used, and avoid the side
effects when this/internal component is not going to be used.

This is a little less clean than I would have liked, but because of
Autoconf's oddity about its AC_CONFIG_* macros, this is the only
solution I could come up with.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

opal/mca/hwloc/hwloc201/configure.m4

@@ -69,27 +69,23 @@ AC_DEFUN([MCA_opal_hwloc_hwloc201_POST_CONFIG],[
 # MCA_hwloc_hwloc201_CONFIG([action-if-found], [action-if-not-found])
 # --------------------------------------------------------------------
 AC_DEFUN([MCA_opal_hwloc_hwloc201_CONFIG],[
+    # Hwloc needs to know if we have Verbs support
+    AC_REQUIRE([OPAL_CHECK_VERBS_DIR])
+
+    AC_CONFIG_FILES([opal/mca/hwloc/hwloc201/Makefile])
+
+    OPAL_VAR_SCOPE_PUSH([HWLOC_VERSION opal_hwloc_hwloc201_save_CPPFLAGS opal_hwloc_hwloc201_save_LDFLAGS opal_hwloc_hwloc201_save_LIBS opal_hwloc_hwloc201_save_cairo opal_hwloc_hwloc201_save_xml opal_hwloc_hwloc201_save_mode opal_hwloc_hwloc201_basedir opal_hwloc_hwloc201_file opal_hwloc_hwloc201_save_cflags CPPFLAGS_save LIBS_save opal_hwloc_external])
+
     # We know that the external hwloc component will be configured
     # before this one because of its priority.  This component is only
     # needed if the external component was not successful in selecting
-    # itself.
+    # itself.  Print out a message explaining this.
     AC_MSG_CHECKING([if hwloc external component succeeded])
     AS_IF([test "$opal_hwloc_external_support" = "yes"],
           [AC_MSG_RESULT([yes])
-           AC_MSG_NOTICE([hwloc:external succeeded, so this component will be skipped])
-           $2],
+           AC_MSG_NOTICE([hwloc:external succeeded, so this component will be configured, but then will be skipped])],
           [AC_MSG_RESULT([no])
-           AC_MSG_NOTICE([hwloc:external failed, so this component will be used])
-           MCA_opal_hwloc_hwloc201_BACKEND_CONFIG($1, $2)])
-])
-
-AC_DEFUN([MCA_opal_hwloc_hwloc201_BACKEND_CONFIG],[
-    # Hwloc needs to know if we have Verbs support
-    AC_REQUIRE([OPAL_CHECK_VERBS_DIR])
-
-    AC_CONFIG_FILES([opal/mca/hwloc/hwloc201/Makefile])
-
-    OPAL_VAR_SCOPE_PUSH([HWLOC_VERSION opal_hwloc_hwloc201_save_CPPFLAGS opal_hwloc_hwloc201_save_LDFLAGS opal_hwloc_hwloc201_save_LIBS opal_hwloc_hwloc201_save_cairo opal_hwloc_hwloc201_save_xml opal_hwloc_hwloc201_save_mode opal_hwloc_hwloc201_basedir opal_hwloc_hwloc201_file opal_hwloc_hwloc201_save_cflags CPPFLAGS_save LIBS_save opal_hwloc_external])
+           AC_MSG_NOTICE([hwloc:external failed, so this component will be used])])
 
     # default to this component not providing support
     opal_hwloc_hwloc201_basedir=opal/mca/hwloc/hwloc201
@@ -99,8 +95,6 @@ AC_DEFUN([MCA_opal_hwloc_hwloc201_BACKEND_CONFIG],[
     opal_hwloc_hwloc201_save_LDFLAGS=$LDFLAGS
     opal_hwloc_hwloc201_save_LIBS=$LIBS
 
-    HWLOC_SET_SYMBOL_PREFIX([opal_hwloc201_])
-
     # save XML or graphical options
     opal_hwloc_hwloc201_save_cairo=$enable_cairo
     opal_hwloc_hwloc201_save_xml=$enable_xml
@@ -143,6 +137,12 @@ AC_DEFUN([MCA_opal_hwloc_hwloc201_BACKEND_CONFIG],[
     AS_IF([test -n "$opal_datatype_cuda_CPPFLAGS"],
           [CPPFLAGS="$CPPFLAGS $opal_datatype_cuda_CPPFLAGS"])
 
+    # Only set the symbol prefix if this component is being used
+    # (i.e., if the external component is not being used).
+    AS_IF([test "$opal_hwloc_external_support" = "no"],
+          [HWLOC_SET_SYMBOL_PREFIX([opal_hwloc201_])])
+
+    # Do the bulk of the hwloc core setup
     HWLOC_SETUP_CORE([opal/mca/hwloc/hwloc201/hwloc],
               [AC_MSG_CHECKING([whether hwloc configure succeeded])
                AC_MSG_RESULT([yes])
@@ -202,8 +202,8 @@ AC_DEFUN([MCA_opal_hwloc_hwloc201_BACKEND_CONFIG],[
     # infrastructure is setup properly (e.g., w.r.t. SUBDIRS=hwloc in
     # this directory's Makefile.am, we still need the Autotools "make
     # distclean" infrastructure to work properly).
-    AS_IF([test "$opal_hwloc_external" = "yes"],
-          [AC_MSG_WARN([using an external hwloc; disqualifying this component])
+    AS_IF([test "$opal_hwloc_external_support" = "yes"],
+          [AC_MSG_NOTICE([using an external hwloc; disqualifying this component])
            opal_hwloc_hwloc201_support=no],
           [AC_DEFINE([HAVE_DECL_HWLOC_OBJ_OSDEV_COPROC], [1])
            AC_DEFINE([HAVE_HWLOC_TOPOLOGY_DUP], [1])])