hwloc/external: improve the error message

Make it clear that the v2.x series does not and will not support hwloc
>= v2.0, and that the user can choose to use Open MPI >= v3.1 if they
need hwloc >= v2.0.

Signed-off-by: Jeff Squyres <[email protected]>

Author: jsquyres

opal/mca/hwloc/external/configure.m4

@@ -189,7 +189,8 @@ AC_DEFUN([MCA_opal_hwloc_external_CONFIG],[
                    ]])],
                [AC_MSG_RESULT([yes])],
                [AC_MSG_RESULT([no])
-                AC_MSG_ERROR([OMPI does not currently support hwloc v2 API
+                AC_MSG_ERROR([The Open MPI v2.1.x series does not support hwloc >= v2.0.
+Open MPI v3.1.0 and later support hwloc >= v2.0.
 Cannot continue])])
            AS_IF([test "$opal_hwloc_dir" != ""],
                  [CFLAGS=$opal_hwloc_external_CFLAGS_save])