[vcpkg|toolchain] add VCPKG_OVERLAY_(PORTS|TRIPLETS) to the toolchain

[Neumann-A]

So people can add the overlays from cmake instead of modifying the environment

[ras0219]

We hope to address this via settings in the manifest file; adding these settings via cmake is problematic because they won't be present when you run other commands like vcpkg search or vcpkg update.

[Neumann-A]

We hope to address this via settings in the manifest file

this does not belong there ..... users should always be free to supply their own version of dependencies and have it not hardcoded! The same goes for toolchain/triplet settings.

via cmake is problematic because they won't be present when you run other commands

the manifest installed dir does not directly belong to the user but to the project and is put into ${CMAKE_BINARY_DIR}/vcpkg_installed as such it will not affect other commands.
Furthermore I can overwrite the corresponding env variable and get a similar behavior. Since messing with env variables within a cmake script is kind of messy I would rather like those to be true cmake variables.

[strega-nil]

@Neumann-A the plan was to put it in the vcpkg-configuration.json file. This would allow overlay ports and triplets to be provided no matter what mode a person is using vcpkg in.

[Neumann-A]

@strega-nil: As far as I understand vcpkg-configuration.json as well as vcpkg.json are meant for the project/library not for the user to mess with.
Putting more stuff into vcpkg.exe instead of cmake also seems wrong from my perspective. I want to have full control from the CMakeLists.txt not from some other location.

[strega-nil]

@Neumann-A the vcpkg-configuration.json is absolutely meant for the user to mess with, we just don't have the capabilities for it yet because we haven't had the time to do the work just yet. Specifically, I'd like to add a vcpkg-configuration.user.json that would allow the user to change settings.

[Neumann-A]

is absolutely meant for the user to mess with

In that case it does not belong into the CMAKE_SOURCE_DIR. Either way you probably will need a cmake variable to hold the path to a user supplied vcpkg-configuration.json in this case.

[strega-nil]

fair enough, I guess.

[strega-nil]

Hrm, I've changed my opinion on this after some cajoling from Billy. I think this feature should be merged.

[strega-nil]

Just the one minor suggestion (which doesn't need to be taken, I just think it might be a nicer design as a whole)