Revert "No QTC_PREFIX default on Windows. Causes too many funny side effects."

Because the change depends on reverted commit
c29bf6f6

This reverts commit 550f8218.

Change-Id: If287c4b126b1ef710c074fd85946da41161c6ba9
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>

qtcreator.pri

@@ -5,6 +5,7 @@ QTCREATOR_VERSION = 2.5.81
 
 isEmpty(QTC_PREFIX) {
   unix: QTC_PREFIX = /usr/local
+  else: QTC_PREFIX = QtCreator
 }
 
 defineReplace(cleanPath) {