ompi_check_lustre.m4: abort if Lustre requested and not found

Follow the OMPI bias: if a human requests feature X and configure
can't deliver feature X, abort and let the human figure it out.

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

config/ompi_check_lustre.m4

@@ -87,6 +87,6 @@ OPAL_LOG_COMMAND(
     AS_IF([test "$ompi_check_lustre_happy" = "yes"],
           [$2],
           [AS_IF([test ! -z "$with_lustre" && test "$with_lustre" != "no"],
-                  [echo LUSTRE support not found])
+                 [AC_MSG_ERROR([Lustre support requested but not found.  Aborting])])
            $3])
 ])