[Bug 58209] add an environmental variable to control the Windows Version

[WineHQ Bugzilla]

http://bugs.winehq.org/show_bug.cgi?id=58209

--- Comment #1 from Nikolay Sivov  ---
You'll have to relaunch anyway, but you can use "winecfg /v winxp" from command
line already, to avoid going through UI.

Then you can configure different version reported for a given exe file you run,
if you need this change set permanently for some application.