Feature Request: Give me a way to specify the MSVC Toolset when using "-G Ninja"

Usage
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9

@steven-johnson

The following module may be useful to discover the MSVC toolchain:

We use it to find the appropriate vcvars script and build external projects that do not use CMake.

A simple CMake script could be created that would use this module and allow you to do this:

cmake -P /path/to/ConfigureWithVisualStudioToolchain.cmake  ^
  -DGENERATOR=Ninja ^
  -DPROJECT_SOURCE_DIR=/path/to/src ^
  -DPROJECT_BINARY_DIR=/path/to/bld

To select a specific visual studio toolchain, you would pass options like -DVcvars_MSVC_VERSION=... -DVcvars_MSVC_ARCH= ....

The script ConfigureWithVisualStudioToolchain.cmake would be something like this:

Note: I did not test the following code

# Sanity check
# TODO: Check that expected parameters are set ... 

set(download_dir ${PROJECT_BINARY_DIR}/CMakeFiles/FindVcvars)
file(MAKE_DIRECTORY ${download_dir})

# Download FindVcvars.cmake
set(dest_file "${download_dir}/FindVcvars.cmake")
set(expected_hash "cacfcb6e243e4a0fa628c5873d0cb444c08079b7729af24093a59712b01b6ff6")
set(url "https://raw.githubusercontent.com/scikit-build/cmake-FindVcvars/v1.3/FindVcvars.cmake")
if(NOT EXISTS ${dest_file})
  file(DOWNLOAD ${url} ${dest_file} EXPECTED_HASH SHA256=${expected_hash})
else()
  file(SHA256 ${dest_file} current_hash)
  if(NOT ${current_hash} STREQUAL ${expected_hash})
    file(DOWNLOAD ${url} ${dest_file} EXPECTED_HASH SHA256=${expected_hash})
  endif()
endif()

list(INSERT CMAKE_MODULE_PATH 0 ${download_dir})
find_package(Vcvars REQUIRED)

# TODO: Support passing argument like toolset, ... 

execute_process(COMMAND
  ${Vcvars_LAUNCHER} ${CMAKE_COMMAND} -G ${GENERATOR} -S ${PROJECT_SOURCE_DIR} -B ${PROJECT_BINARY_DIR}
  )