Move decision logic about windows/header generation closer to install_scripts, as it doesn't appear to be used elsewhere.

Skip integration tests when one of the packages under test is already installed, as the installation will fail in that case.

Remove dependence of the script on its own file, but instead generate the necessary file in a temporary directory.

Test is passing when LC_CTYPE isn't present. According to the original ticket, Distribute 103, this test was only failing for "C" and "POSIX", so only expect the failure there.