This lets the user select if system or builtin libffi version shall be used.
Default is still "try system libffi and fallback to builtin".

The option can be used like so:
    gem install ffi -- --disable-system-libffi

Primary use is to run specs on builtin libffi on Windows in addition to
system libffi.

rake-10.5.0/lib/rake/application.rb:381: warning: deprecated Object#=~ is called on Proc; it always returns nil

... and use it in Appveyor-CI.

This temporary switches to a forked libffi, until the fixing PR is merged:
  libffi/libffi#465

Use the following command to update the submodule:
  git submodule update --init --remote

PipeHelperWindows.c:15:5: warning: implicit declaration of function
'sprintf' [-Wimplicit-function-declaration]
     sprintf( name, "\\\\.\\Pipe\\pipeHelper-%u-%i",
          ^~~~~~~