add webrtc-audio-coding public library

This new lib contains the bare minimum to implement an iSAC encoder and
decoder.

The webrtc files have been copied from the revision as the existing
imported files (c8b569e0a7ad0b369e15f0197b3a558699ec8efa).

meson.build

@@ -111,3 +111,15 @@ pkgconfig.generate(
     ] + platform_cflags,
     libraries: libwebrtc_audio_processing,
 )
+
+pkgconfig.generate(
+    name: 'webrtc-audio-coding',
+    description: 'WebRTC Audio Coding library',
+    version: meson.project_version(),
+    filebase: 'webrtc-audio-coding',
+    subdirs: 'webrtc_audio_processing',
+    extra_cflags: [
+      '-DWEBRTC_AUDIO_PROCESSING_ONLY_BUILD',
+    ] + platform_cflags,
+    libraries: libwebrtc_audio_coding,
+)