Avoid qtbase/bin/bin artifact when running make install

Fix regression introduced in commit 63660402

. exists()
also returns true for a directory ...

Change-Id: I2b4fff00b18eeba53e959306ab33c3bef3795987
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

qtbase.pro

@@ -49,7 +49,7 @@ INSTALLS += qmake
 #licheck
 licheck.path = $$[QT_HOST_BINS]
 licheck.files = $$PWD/bin/$$QT_LICHECK
-exists($$licheck.files): INSTALLS += licheck
+!isEmpty(QT_LICHECK): INSTALLS += licheck
 
 #syncqt
 syncqt.path = $$[QT_HOST_BINS]