Build-system fixups to install more headers, add a missing absl dependency, and forward port some missing patches to fix Windows builds.

Bump to code from WebRTC M131 version.

Changes include:

• Minor (breaking) API changes upstream
• Various improvements to the AEC implementation
• Transient suppression is removed
• ExperimentalAgc and ExperimentalNs are removed
• iSAC and the webrtc-audio-coding library were removed
• abseil-cpp dependency bumped to 20240722
• NEON runtime detection dropped following upstream
• Fixes for building on i686 and MIPS
• Support for BSDs is added
• Other build-system cleanups
• Patches to upstream are now also tracked in patches/