Fixed waf building on OSX.

This makes sure header checks work when the third party package is
installed in a non-standard location, as long as a pkg-config file is
found.

It is already defined by conf.define().

Release audio devices when alsa_driver_new fails

Call JackAlsaDriver::Close() when alsa_driver_new() fails, to
release devices acquired just before that call. Otherwise
JackAlsaDriver::Open() will never succeed again, as re-acquiring the
same device will fail with "A handler is already registered" DBus error.

fix missing void parameter in prototypes

Fix CPU hogging of the midi_thread()

The midi thread was always late after 2^31-1 Samples (~13.5h at 44.1kHz), because when there's no time limit, a uint64_t sample counter was compared against a signed 32bit INT_MAX to determine lateness. Now the CPU lockup will occur only after ~3 million years (at 192kHz), and because of the overflow it will presumably fix itself after a few milliseconds.

Fix 3 minor issues.

It's used for get_cycles() implementation, but it's never used.
On certain architectures it gives a warning which is be confusing.