Apply warning flags to RemoteVstPlugin too

[DomClark]

Non-native versions of RemoteVstPlugin are built using CMake's ExternalProject module, so don't pick up flags and things from the main project. This PR ensures that we apply the warning/error flags consistently to all our code.

Changes:

• Pull the warning/error flag code out into a separate CMake module, so it can be included in both the main project and "external" projects.
• Bump the CMake version for RemoteVstPlugin to match that of the main project.
• Forward USE_WERROR to RemoteVstPlugin.
• Fix warnings currently emitted for RemoteVstPlugin (one fix for each of unused-variable and sign-compare).

[Rossmaxx]

Looks good.