Use TAP automake driver when possible.

With automake 1.12, it's possible to use a custom test driver, and so we
can use its TAP driver instead. When the sass-spec framework is set up
to output TAP, then the automake driver will understand all the subtests
and be able to count them individually.

Note, I'm not sure if this conditional function is GNU make specific or
not. Unfortunately, it seems to be the only way to check if we're using
automake 1.11.