- make the mutex itself static data (not part of the exported api)
- export lock/unlock functions unconditionally (but they're no-ops if locking and/or fftw is not enabled)
- those first two changes allow the swig bindings to expose the mutex reliably
- rename the preprocessor macros wrapping the lock/unlock functions to better reflect their purpose
Original: 755e4d0d1f3caa425abc34a1ab1a07ce8344d63d