Fix for detecting wince build

Need to use QMAKE_XSPEC, not QMAKE_SPEC for 'qmake -query' in Qt5
to get correct target mkspec.

Task-number: QTVSADDINBUG-176
Change-Id: I0f643a8e8a79ecdac893c441342b05d9fab80a95
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>