Windows: Fix PowerShell 2.0 compatibility.

PowerShell 2.0's Get-ChildItem doesn't support "-File" even though
http://technet.microsoft.com/en-us/library/hh847897.aspx says otherwise.

Add a config.nmake option to test PS 2.0 compatibility.

Change-Id: I8de83cd0841d4ab0b01351c2d39084578a2c84ee
Reviewed-on: https://code.wireshark.org/review/3625
Reviewed-by: Gerald Combs <gerald@wireshark.org>