Update Speex AEC to the latest version to get multichannel EC

[bennylp]

Ticket #588 rolled-back Speex AEC to beta3 version which does not support multi-channel EC, so we need to update it to the latest version to support multi-channel.

Testing is done with pjsua on Mac with the following setups: No echo cancellation at all (ec-tail = 0), stereo mode, and mono mode.

For stereo mode, you can clearly observe some echo cancellation compared to the testing without ec, however you can also still notice some uncancelled echo. And this is especially more noticeable after you compare it with mono testing, where the AEC seems to work pretty well and manages to cancel most of the echo.

Note that this ticket only updates the Speex DSP part to version 1.2, but leaves the codec part untouched.

[ming]

In 6129:

Closed #589: Update Speex AEC to the latest version to get multichannel EC

[ming]

In 6130:

Re #589: Fixed missing include file

[riza]

In 6135:

Re #589: Add scal.c to Visual Studio project files.

[ming]

In 6143:

Re #589: Suppresses warning of unused var