Support webrtc-aec3 on windows

[trengginas]

PR #2722 add support for WebRTC AEC3.
This ticket will add the support for Visual Studio (windows).

Requirement:

• Visual Studio 2015 or later with C++17 support

Changes:

• Some source files needs to be renamed. Visual Studio will choose one implementation if source code are using the same name, which might lead to linker error.
• Create new project to Visual Studio solution (libwebrtc_aec3). By default it is not build since it will add considerable time to build. e.g: On my machine it will take ~2 minutes to build the single project. Compared to ~50s to build all the projects (without libwebrtc_aec3) in the solution.
• Define some build macro specific to building WebRTC-AEC3 on windows.
• Add echo_webrtc_aec3.c to pjmedia project.

Enabling WebRTC-AEC3 on Visual Studio 2015 or later:

1. Define PJMEDIA_HAS_WEBRTC_AEC3 on config_site.h file
2. Change the BuildToolset configuration on build/vs/pjproject-vs14-common-config.props to v142.

     <When Condition="'$(API_Family)'=='WinDesktop'">
       <PropertyGroup>
-        <BuildToolset>v140</BuildToolset>
+        <BuildToolset>v142</BuildToolset>

3. Enable buildng libwebrtc-aec3 project from solution configuration manager.
4. Add reference to libwebrtc-aec3 project on libpjproject project. On libpjproject project > References > add libwebrtc-aec3 project.
5. When building samples project, you need to link with libwebrtc_aec3 library by editing Samples-vc.mak file.

+WEBRTC_AEC3_LIB = ..\..\third_party\lib\libwebrtc_aec3-$(TARGET)$(LIBEXT)
 
 THIRD_PARTY_LIBS = $(GSM_LIB) $(ILBC_LIB) $(RESAMPLE_LIB) \
 		   $(SPEEX_LIB) $(SRTP_LIB) $(G7221_LIB) $(BASECLASSES_LIB) \
-		   $(YUV_LIB) $(WEBRTC_LIB)
+		   $(YUV_LIB) $(WEBRTC_LIB) $(WEBRTC_AEC3_LIB)

[sauwming]

Looks okay. Please add the modification to WebRtc's original source in PJSIP_NOTES. Something like this:
master...webrtc-aec3-2#diff-f88a8593b35ff47a063c4fcfadd61ceb5c2fc83b4a411756269b03ebe19a646a

[austinkottke]

Will we support a mingw64/32 compile?