help: try to fix windows case for qdoc detection

doc/doc.pri

-QDOC_BIN = $$(QTDIR)/bin/qdoc3
+unix:QDOC_BIN = $$(QTDIR)/bin/qdoc3
+win32:QDOC_BIN = $$(QTDIR)/bin/qdoc3.exe
 win32:QDOC_BIN = $$replace(QDOC_BIN, "/", "\\")
 
+# legacy branch, can be dropped as soon as we depend on Qt 4.6
 !exists( $$QDOC_BIN ) {
-	QDOC_BIN = $$(QTDIR)/tools/qdoc3/qdoc3
-	win32:QDOC_BIN = $$replace(QDOC_BIN, "/", "\\")
+    unix:QDOC_BIN = $$(QTDIR)/tools/qdoc3/qdoc3
+    win32 { 
+        QDOC_BIN = $$(QTDIR)/tools/qdoc3/release/qdoc3.exe
+        QDOC_BIN = $$replace(QDOC_BIN, "/", "\\")
+        !exists( $$QDOC_BIN ) {
+            QDOC_BIN = $$(QTDIR)/tools/qdoc3/debug/qdoc3.exe
+            QDOC_BIN = $$replace(QDOC_BIN, "/", "\\")
+        }
+    }
 }
 
 unix {