Add support for iOS SDK 7 & iOS 7

[ming]

This ticket will add support with regard to changes in iOS SDK 7 and iOS 7, such as:

• Deprecated C-interface Audio Session API.
  PJSIP now uses AVAudioSession to set audio session category and activate/deactivate audio session. Application will need to do its own audio session management to handle input/output route and notifications (interruption, media server reset, etc). Please refer to ​Apple's official doc on Audio Session Programming Guide.

• Audio volume control with VoiceProcessingIO Audio Unit
  It is reported that when using VoiceProcessingIO audio unit on iOS 7, there are a few issues:
  • Unable to mute the volume using the side volume button (the minimum volume is one bar). This didn't happen before iOS 7.
  • Unable to synchronize with the control center volume slide bar. Since control center is a new feature in iOS 7 we cannot compare it with pre-iOS 7.
  • After the teardown of the audio unit, AVAudioPlayer will play at a reduced volume if the category is set to PlayAndRecord with category option DefaultToSpeaker. (Update: According to Apple's reply, this is due to the gain controller feature employed in VPIO, which still applies even after its uninitialization. There is no API or workaround to change the gain controller but it is reported that the issue is being investigated as to whether the gain controller affect the playback volume too drastically.)
  The issue does not happen with RemoteIO (which can be used by disabling the EC). Testing with aurioTouch2 sample app provided by Apple showed the same result, so this is likely Apple's issue.
  Currently there is no resolution or workaround other than disabling EC.

[ming]

In 4590:

Re #1697: Add internal compile time option to disable the use of Audio Session API (deprecated)

[ming]

In 4610:

Re #1697: Add device info in pj_get_sys_info() and set default clock rate for iPhone 5 running iOS7 to 44100Hz

[ming]

In 4617:

Re #1697: Use AVAudioSession to set audio session category and activate/deactivate audio session

[bennylp]

In 4635:

Re #1697: fixed build error with the new coreaudio_dev.m introduced by r4617, because build process looks for an ObjC compiler

[ming]

In 4636:

Re #1697: Removed extra spaces in aconfigure.ac