This commit:
- Switches from Ubuntu 16.04 to 18.04 for all builds, mainly in order to use a more recent version of libsodium.
- Installs libsodium, used by the WireGuard implementation.
AppVeyor provides vcpkg and a great guide explaining how to use it and cache the installed packages: https://www.appveyor.com/docs/lang/cpp/#vc-packaging-tool
However, we're currently relying on Azure Pipelines for the Windows builds and keeping one on AppVeyor would be a waste of CI resources.
The purpose of the configuration was to build installers and continually verify that MSVC 2008 could still build the project.
MSVC 2008 was a requirement because we wanted to maintain support for very old versions of Windows (9x).
During the past few years we encountered many annoying limitations due to that requirement, such as the lack of support for designated initializers.
We ended up deciding to only keep support for operating systems older than 7 in the stable repository.
As for the installers, we now build them on Azure Pipelines (currently with MSVC 2019).
