avoid that CROSS_COMPILE affects host builds

Change-Id: Iee40cfd87d7ec640fb4b3526e58317bdc8a778c7
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: David Schulz <david.schulz@digia.com>

mkspecs/features/device_config.prf

@@ -5,7 +5,9 @@ DEVICE_PRI = $$[QT_HOST_DATA/get]/mkspecs/qdevice.pri
 exists($$DEVICE_PRI):include($$DEVICE_PRI)
 unset(DEVICE_PRI)
 
-isEmpty(CROSS_COMPILE) {
+host_build {
+    CROSS_COMPILE =
+} else: isEmpty(CROSS_COMPILE) {
     #this variable can be persisted via qmake -set CROSS_COMPILE /foo
     CROSS_COMPILE = $$[CROSS_COMPILE]
 }