Fix playback when framerate changes

- When playing FFMPEG-decoded videos if the video stream
  framerate changed there was some automatic timing
  adjustment code that would set that frame's time
  to what was "expected". This code comes from
  26c79dbf31809d71235eb5871834d5a65f5dc314
  (fixed the problem that some mkv file is played not
  smoothly after seeking). Since the commit doesn't
  really explain the problem and I'm seeing it cause
  a real problem in this case I'm deleting the code.
  If the problem described in that commit returns
  then the issue can be reevaluated taking this new
  issue into consideration.
- What would happen is that when the framerate changed
  libav would deliver a frame that was (in my case)
  slightly farther ahead in time than we expected the
  next frame to be. So we would force the frame's time
  to be what we actually expected the next frame's
  time to be and picked a new next frame time based
  on that time. This would cause frames to eventually
  get far enough out of sync that the timer adjustment
  code in the display path would see us way out of
  sync and force the clock current time to jump forward.
  This would make the video play as though it were
  being fast-forwarded.
Update project to work with new Xcode on 10.9

- Update compiler to use the default. I'm not too excited
  about this as it might cause trouble with people building
  on different versions of Xcode but trying to carry obsolete
  compilers forward seems worse. In practice I'm sure this
  will work fine.
- Set deployment target to 10.7. Not sure why I had it at 10.8
  before but whatever. I'd like to support as old as is
  practical right now and 10.7 is it for now.
- Set it to build against latest SDK since Apple is aggressive
  about removing old SDKs. Just have to be careful about using
  the right API calls.
Fix compile errors found with Xcode on 10.9

- Missing OpenGL header imports.
Specifically specify clang++ for building Matroska

- trying to use GCC was getting us mismatched standard c++
  libs which caused link in Movist.
- delete contrib/build if you're getting link errors.