Default mkspec is not installed for shaddow build of Qt

Test case
1) create a shadow build directory, eg \build\qt\4.7
2) change into directory \build\qt\4.7
3) configure Qt with at least -prefix \Qt\4.7
4) run nmake install_mkspec
5) confirm that directory \Qt\4.7\mkspecs\default exists

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Ideally check that the non-shadow build Qt works as expected,eg
1) Remove directory \Qt\4.7\mkspecs\default
2) extract Qt 4.7 source package into \build\qt\4.7_source
3) change into directory \build\qt\4.7_source
4) configure Qt with at least -prefix \Qt\4.7
5) run nmake install_mkspec
6) confirm that directory \Qt\4.7\mkspecs\default exists