Running a bdist_wininst installer, built with Python 2.3, installing

for Python 2.4 caused a segfault when post_install_script was used.

The reason was that the file handle passed to PyRun_SimpleFile() was
created with MSVCRT.DLL, but Python 2.4 uses MSVCR71.DLL.

So, I replaced PyRun_SimpleFile() with PyRun_SimpleString().  The
segfault is gone, but the output of the postinstall script doesn't
show up, because still freopen() from MSVCRT is used.

Already backported.