Avoid cmake_minimum_required(VERSION ...)- usage

[MarkSchofield]

Fixes: #167

WindowsToolchain attempts to enforce a minimum version of CMake by writing:

cmake_minimum_required(VERSION 3.20)

But that both enforces the CMake version, and applies policies - which is inappropriate; only a consuming build should do that. This PR replaces cmake_minimum_required-calls with:

if(CMAKE_VERSION VERSION_LESS 3.20)
    message(FATAL_ERROR "WindowsToolchain requires at least CMake 3.20, but CMake ${CMAKE_VERSION} is in use.")
endif()