in  toolset command Expand Environment Variables

I can use "%APPDATA%" in set command.

```
    "golang":
    [
	{
	    "path":
	    [
		"C:\\opt\\go\\bin"
	    ],
	    "set":
	    { "GOPATH": "%APPDATA%\\gocode" }
	}
    ],
```
Merge pull request #1 from takekazuomi/patch-1

in  toolset command Expand Environment Variables

Allows environment variables to depend on others that already exist.