use the right reference variable for source path

$$PWD is not really defined at this point. in qt4 it was the last value
assigned to it, while in qt5 it is just missing.

Change-Id: I4e70d234a7d5cee88e2d081b053e1ffa1070f788
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>

qtcreator.pri

@@ -65,7 +65,7 @@ defineReplace(stripSrcDir) {
         !contains(1, ^/.*):1 = $$OUT_PWD/$$1
     }
     out = $$cleanPath($$1)
-    out ~= s|^$$re_escape($$PWD/)||$$i_flag
+    out ~= s|^$$re_escape($$_PRO_FILE_PWD_/)||$$i_flag
     return($$out)
 }