Setting specific WindowsPlatform

[TheJare]

The toolchain script currently only allows configuring the windowsTargetPlatformVersion project setting if the toolset target is set to "--vs=winstore82". Otherwise the project setting will default to 8.1. It seems this parameter should be settable always. Three suggestions (can't make a pull request right now):

1. Move the 3 lines that assign the platform, from the "winstore82" subsection up to the enclosing if, right before starting to check specific "--vs" toolsets.

    or _ACTION == "vs2013"
    or _ACTION == "vs2015"
    or _ACTION == "vs2017"
    then
   
    	local action = premake.action.current()
    	action.vstudio.windowsTargetPlatformVersion = windowsPlatform
    	action.vstudio.windowsTargetPlatformMinVersion = windowsPlatform
   
    if (_ACTION .. "-clang") == _OPTIONS["vs"] then
   

2. Allow a smarter default for the "--with-windows" argument that sets the "windowsPlatform" variable, based on the standard environment variable "WindowsSDKVersion", and in its absence, default to a more conservative platform of 8.1.

    local windowsPlatform = "8.1"
    if os.getenv("WindowsSDKVersion") then
    	windowsPlatform = os.getenv("WindowsSDKVersion")
    end
    if _OPTIONS["with-windows"] then
    	windowsPlatform = _OPTIONS["with-windows"]
    end
   

Perhaps the various toolsets may enforce higher values for this setting, so for example if requesting winstore82 then convert 8.1 into the previous default of 10.0.10240.0

3. Update the newoption for with-windows to describe the new default more accurately.

    newoption {
    	trigger = "with-windows",
    	value = "#",
    	description = "Set the Windows target platform version (default: $WindowsSDKVersion or 8.1).",
    }
   

[bkaradzic]

I'll add option 2, 3. WindowsSDKVersion environment variable is way to go.