Log Message:
Update to 1.3.1

Changelog:
Apr 12, 2019
This Opus 1.3.1 minor release 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).