VS 2019 does not default to the 8.1 SDK as VS 2017 and VS 2015 did.
When `CMAKE_SYSTEM_VERSION` is 8.1 or lower, select the 8.1 SDK
explicitly.
Fixes: #18927
The check added by commit 0a29a31161 (VS2017: Verify Windows 8.1 SDK
before using it, 2017-04-25, v3.8.1~2^2) used the wrong path to
`windows.h` within the SDK, leading to it never being detected.
Fixes: #18923
VS 2017 and VS 2019 provide `amd64/MSBuild.exe` variants next to
their `MSBuild.exe` tools. When the 64-bit host toolchain is
selected (e.g. via `host=x64`), select the 64-bit MSBuild too.
Fixes: #18219
Add a `cmGlobalGeneratorFactory::GetKnownPlatforms` method to return
a list of known possible values for `CMAKE_GENERATOR_PLATFORM`.
Implement the method for each generator by referencing the list of
possible values documented in `Help/generator/*.rst` for it.
Co-Author: Julien Jomier <julien.jomier@kitware.com>
Replace `cmGlobalGeneratorFactory::GetGenerators` with a pair of methods
to split the list of generator names into those that have platforms in
the name and those that do not.
Add this generator *without* support for specifying the target
architecture in the generator name. cmake-gui will be taught
to provide a field for this, and command-line builds can use -A.
Also, teach this generator to select a default target architecture
based on the host architecture.
Fixes: #18689
Inspired-by: Egor Pugin <egor.pugin@gmail.com>
