We now use a pre-built libffi binary from our binaries repository, and no longer vendor the full implementation.

Also enable building externals by default on Windows, use PCbuild\build.bat's -E option to disable it.

* bpo-30450: Fall back to git.exe if no Python is found.

* Also check whether git.exe is on PATH if it will be used.

Updates ssl and tkinter projects to use pre-built externals

The Windows build now depends on Python 3.6 to fetch externals, but it will be downloaded via NuGet (which is downloaded via PowerShell) if it is not available via `py -3.6`.  This means the only thing that must be installed on a modern Windows box to do a full build of CPython with all extensions is Visual Studio.

Also fixes an outdated note about _lzma in PCbuild/readme.txt

Updates checked-in line endings on several files.

Without this option, AppVeyor log is too unreadable.

Don't download sources that won't be used.

Triggers clean on buildbots.

This is mostly a backport of issue #21907, but also includes a few
extras necessary to make the bulidbot scripts as thin as possible.