Building static libs with mingw32-make and Git Bash generates NUL files

Since this commit http://code.qt.io/cgit/qt/qtbase.git/commit/?id=f5f772849ca1ada44f3de537633eec2664b14fd4, qmake deletes .a files before calling ar. When building on Windows with mingw32-make from the Git Bash shell, this generates a command like this:

rm foo.a 2>NUL

This causes the creation of a NUL file instead of suppressing the error message.

I believe the code should use the isWindowsShell() function to decide whether it should redirect to NUL or to /dev/null instead of hardcoding it at build time.