Add ability to disable or force system libffi and fix stdcall on Win32

[larskanis]

This makes sure that we run tests with system libffi and with bunded libffi on Appveyor (Windows build). The specs fail on current libffi master branch due to broken stdcall to closures. This can be seen here.

The failing stdcall issue is fixed by this PR. This is why the PR enables a libffi fork temporary.

Fixes #649

[tduehr]

Can you split this into multiple PRs?

[larskanis]

@tduehr I already separated the changes into different commits but they belong semantically together. Which objections do you have to merge this?

[tduehr]

The only real issue I have is using a libffi fork. Does FFI not work on windows at all without it?

[larskanis]

The only real issue I have is using a libffi fork.

Uh, that's the simplest way to patch the upstream library. This is also common practice, that core members provide forked repositories in order to apply patches, until the particular issue has been fixed upstream. See here in rails for example. But I can move it to https://github.com/ffi/libffi/ if you like.

Does FFI not work on windows at all without it?

As mentioned above, it fails at this one test case I wrote years ago. But this test case wasn't triggered on Appveyor with system libffi in the past.

[eregon]

FWIW, I had this in my .git/config:

[submodule "ext/ffi_c/libffi"]
	active = true
	url = https://github.com/libffi/libffi.git

I tried removing it, and rm -rf ext/ffi_c/libffi, but even after that git would still not clone from the correct repository with git submodule update --init --remote:

$ git submodule update --init  --remote
fatal: Needed a single revision
Unable to find current origin/fix-stdcall revision in submodule path 'ext/ffi_c/libffi'

$ git submodule update --init  
error: Server does not allow request for unadvertised object 024800a1331d38794c24cf191cd74b70af1078a1
Fetched in submodule path 'ext/ffi_c/libffi', but it did not contain 024800a1331d38794c24cf191cd74b70af1078a1. Direct fetching of that commit failed.

The only way I found was to:

cd ext/ffi_c/libffi
git remote set-url origin https://github.com/larskanis/libffi.git
git fetch
cd ../../..
git submodule update

Might be a git bug (I use git 2.13.6).
Submodules still looks like a half-implemented feature in git since I last used them, unfortunate.

[larskanis]

@eregon Usually I'm the one who's called in the company when there any git issues, but git submodules are very special and don't fit well to the rest of the git commands. So I don't like them.

Nevertheless I updated two ruby-ffi repositories by git pull && git submodule update without issues. Did you try git submodule update without arguments?

[eregon]

@larskanis Yes, I think I tried that too. Maybe it's a bad interaction with git worktree which I use on this repository to have two copies but share all git data locally.