configure correctly tests dtrace -G in version 1.11

dtrace version SUN D 1.11 introduces a check in the dtrace
compiler to ensure that probes actually exist. If there
are no probes, then the -G step will fail. As this test
is only being used to determine whether -G is necessary
(for instance, on OSX it is not), adding a real probe to
the conftest allows it to succeed on newer versions of dtrace.