Opus 1.3.1

Opus 1.3.1 is a minor release that fixes an issue with the analysis on files
with digital silence (all zeros), especially on x87 builds (mostly affects
32-bit builds). It also includes two new features:
- A new OPUS_GET_IN_DTX query to know if the encoder is in DTX mode (last frame
  was either a comfort noise frame or not encoded at all)
- A new (and still experimental) CMake-based build system that is eventually
  meant to replace the VS2015 build system (the autotools one will stay).