Add a $(BITS) convenience variable

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
aubio: Update

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
aubio: Enable shared build

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
aubio: Add a test program

Signed-off-by: Timothy Gu <timothygu99@gmail.com>