Add support for msvc(portable) in platform expression

[ankurvdev]

Support msvc(portable) by using a separate triplet that avoid a vswhere based search by specifying set(VCPKG_CMAKE_SYSTEM_NAME "Windows")
in the triplet file.

[ankurvdev]

@BillyONeal . Can you please review and start a build ?

[BillyONeal]

Can you describe what this fixes? Empty string == Windows has been widespread CMake behavior for a long time.

[ankurvdev]

The scenario here is probably a little unofficial but I'm using portable-msvc method to download the vs header, libraries and toolchains without actually installing visual studio
There's lots of instructions that explain how to do that like these
https://github.com/Data-Oriented-House/PortableBuildTools
https://gist.github.com/mmozeiko/7f3162ec2988e81e56d5c4e22cde9977
I have my own little python toolchain downloader here

The main benefit of this is that I don't need to install as admin and also helps avoid unnecessary cruft and I can pick and choose multiple versions SxS.
VSCode is easily able to pick it up using just the envvars and so it works for the most part.

The problem with vcpkg is that its actually not the empty == "windows" that is the problem. The issue is if its empty vcpkg-tool uses vswhere to determine where to find the VS installation and simply complains that I dont have visual studio installed.
In this scenario I dont have vswhere installed
Now if I actually pass "Windows" then it simply skips the vswhere detection and I am able to pass across envvars for the rest.

There might be a better way to do this but given that this was a cheap fix I figured I'd try to see if it'd be ok

[ankurvdev]

To clarify my earlier comment its actually the filters not working that is the problem.
So some ports for eg will have dependencies that say !windows and without this the vcpkg-tool does not consider that platform to be windows when I use portable msvc

[Neumann-A]

https://learn.microsoft.com/en-us/vcpkg/users/triplets#vcpkg_load_vcvars_env ?