allow passing values with spaces to -device-option

note that the value is written verbatim to the qmake file, so additional
quoting may be required on the command line.

Task-number: QTBUG-30102
Change-Id: I02ca9a44fae82b6932982e6385508b8a304cc1e7
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

configure

@@ -1419,7 +1419,7 @@ while [ "$#" -gt 0 ]; do
     device-option)
         DEV_VAR=`echo $VAL | sed "s,^\(.*\)=.*,\1,"`
         DEV_VAL=`echo $VAL | sed "s,^.*=\(.*\),\1,"`
-        DeviceVar set $DEV_VAR $DEV_VAL
+        DeviceVar set $DEV_VAR "$DEV_VAL"
         ;;
     qpa)
         QT_QPA_DEFAULT_PLATFORM="$VAL"