Incorporate @ecky-l improvements

[medranocalvo]

Hello @ecky-l (writing the ticket here as there's no "Issues" in your fork). I've seen you've made many improvements to Ffidl in your fork. I'm keen on including them here. Would you like helping in doing that?

[ecky-l]

Hello @medranocalvo ,
Yes fine with me. It's been quite some time that I made those improvements and out of the top of my head I don't know what they actually were about... But feel free to pick from my fork whatever you want and to ask me about the details - on a casual basis I can surely remember :)

[medranocalvo]

Good, thank you. Please, confirm that you license your changes under the BSD and specify how your copyright line should read (e.g. mine was Copyright (c) 2015, Patzschke + Rasp Software GmbH, Wiesbaden).

These are the things I need some help with:

1. Support for building with MSVC.
   I'd like to make it easy for others to build Ffidl. I mostly don't work on Windows, and am at a loss w.r.t. Visual Studio (current beta release is built with MinGW). Would it be possible for you to adapt the Visual Studio solution to the new build system. The main difference is that libffi is no longer included in the repository, and one is supposed to build it separately before building Ffidl. Doing this would also help figuring out whether the build system and instructions (https://prs-de.github.io/ffidl/#install) are sufficient.

2. Win64 support. The new beta release implements support for Win64. I see some work towards that in your fork as well. Could you test the new beta on your Win64 projects? I would be most grateful if you could review the implementation, mostly 5629dae, and c5817f4 (let me know of any questions).

I'm not yet sure about ffidl_pointer_deref. I'd like a more general version of that, where we can define the pointee type of pointer types...

I'll cherry pick the rest.

Thank you again.

[ecky-l]

Ah ok,
Yes the license terms are the same as in the original repo, BSD. As for the copyright, I don't care much but you can take your phrase and my real name (Eckhard Lehmann).

Support for building with MSVC.

The solution files in the win/ subdirectory should work out of the box for MSVC 2015. However to make the build experience as easy as possible I think it would be best to port the whole thing to CMake. That would take some time, sure, but when it's done it provides the best and most modern build experience on any platform.
Btw, I turned libffi into a git submodule, so that is not part of the repository in my fork. You might want to have it that way in the official release as well.

Win64 support

The solution files in the win/ subdirectory build for 32 and 64 bit on windows, given Tcl is installed in 32/64 bit versions (ActiveTcl). It worked for me out of the box with maybe some tweaks in the code of my fork.