#define global constant(s) for client-observable long timeout durations e.g. `PA_TIMEOUT_SECONDS`

[RossBencina]

PR #1166 adds #defines a constant PA_START_TIMEOUT_SECONDS to pa_linux_alsa.c that is the amount of time to wait for ALSA to start.

It might be a good idea to globalize constant(s) for these kind of client-observable wait timeouts. I can't think of a reason why the user should wait 2 seconds for ALSA and 20 seconds for DirectSound.

Having global constants also has the advantage of making it easy to grep for places where PortAudio has large user-observable wait times.