Fix overwriting translation with multiple selection

On Windows, when the user selected multiple entries with
Shift-selection, first selecting an item and then another a few items
_above_ the first one, the first item in the selection (the topmost one)
would have its translation overwritten.

Fix this by not doing anything in UpdateFromTextCtrl() if there's
anything other than a single selection.
Use current OS X SDK in ant build

Instead of hardcoding the SDK version, use the $SDKROOT variable
provided by Xcode.

Fixes compilation with Xcode 6, which no longer ships 10.8 SDK.