Project version is inconsistent with release note

[seungha-yang]

The project version written in cmake file is 3.1.8 but it's 3.18 in release note

[nirbheek]

It would also be good to use the project version number when making tags / releases :)

[JPeterMugaas]

The version should be bumped up to 3.1.9.

[walbourn]

I have some complexities that I have to manage here because this library ships in the windows SDK.

There is no functional change here at all.

For vcpkg, the version is the release date.