JACK input device doesn't link on OS X

[nieder]

This is copied from the old roundup #976
 https://roundup.libav.org/issue976

On OS X, using --enable-indev=jack, the build fails with

CC	libavdevice/jack_audio.o
/src/ffmpeg/libavdevice/jack_audio.c: In function 'audio_read_packet':
/src/ffmpeg/libavdevice/jack_audio.c:277: warning: implicit declaration of function 'sem_timedwait'
AR	libavdevice/libavdevice.a
LD	libavdevice/libavdevice.52.dylib
Undefined symbols:
  "_sem_timedwait", referenced from:
      _audio_read_packet in jack_audio.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
make: *** [libavdevice/libavdevice.52.dylib] Error 1

sem_* is semaphore_* on OS X.

[nieder]

provides semaphore_* when HAVE_MACH_SEMAPHORE_H is set

[cbsrobot]

this patch fixes the issue on osx 10.6.6

[michael]

Fixed by disabling jack when sem_timedwait() is unavailable.
About the patch, i think thread synchronization functions should be used that are available on a wider range of platforms, not tons of #ifdefs to make it work on osx & linux and then still fail on openbsd which the patch would sadly.

[michael]

Rethinking this should be a enhancement request of course, as our jack code should use wider available therad sync functions.