pjsip: Use mach semaphores on OSX

Ignore-this: 6e30b44bab5770e73f0d94c94699d363

"Normal" semaphores are not allowed by Apple's sandbox

darcs-hash:20140917122124-56b4d-7661060682c4357c98514fa4a9601bf9b5cc8f0a
pjsip: add AVFoundation video capture device

Ignore-this: 54154756b411141e37ed12edae691628

Replaces the QTKit implementation

darcs-hash:20140917133403-56b4d-ad25dc6fb2537bbd9b5b39dc5443381d5480491b
Simplify code for obtaining the path to the OSX SDK

Ignore-this: 59d395ed5408c96db541578466fa968d

darcs-hash:20140918104825-56b4d-1b0ebf25ec6dc29ac3b021d4f786989c539a0711