Support MSYS2 MinGW64 builds

[neptoess]

These changes will allow v8go to work for users using the MSYS2 MinGW-w64 toolchain (https://www.msys2.org/) and the v8 package in their package manager,
pacman -S mingw-w64-x86_64-v8

Resolves #7, resolves #44

Note that the snapshot_blob.bin file will have to exist in your program's working directory. If the default install paths are used, this file is located at C:\msys64\mingw64\bin\snapshot_blob.bin

[mwm-tesla]

awesome!

[rogchap]

Amazing @neptoess for getting this working on Windows.

One of the key design decisions of v8go is to include pre-built binaries so that users are able to simply use go get and it "just works".

If we're not able to do this for Windows we'll need to add detailed instructions to the README. Are you able to add this to the PR?

[neptoess]

@rogchap Adding prebuilt isn't a great option on Windows, in my opinion, because the default Go installer won't even leave the user with working cgo. Mingw-w64 gcc is the only compiler I'm aware of that allows creating native Windows binaries via cgo, and MSYS2 is a pretty popular way to acquire it, because of how awesome their pacman repo is. It's one extra command for the user to acquire libv8 in addition to gcc. Now, if this package contained a static build of libv8, I would definitely say we should bundle it here. Because it doesn't, shipping a static libv8 for Windows means signing up to build said static library with the Mingw-w64 toolchain. I took a look at the scripts used to build the package (https://github.com/msys2/MINGW-packages/blob/master/mingw-w64-v8/PKGBUILD) and decided that it would probably be significantly more effort to get a static library built than to just dynamically link V8.

tl;dr Yeah I'll modify the README :)

[neptoess]

@rogchap Let me know how this looks 458d4c7

[rogchap]

Yup; this is a fair compromise; might also be a better way to provide support for other architectures like freebsd etc. So static binaries for darwin and linux, but dynamic for the others 🤷‍♂️

[rogchap]

Thank you. Looks good. Seems I need to fix the CI build (need to merge #6)

[rogchap]

Hey @neptoess Thanks for continuing to update this PR.
I've fixed up the test so I could check for any regressions in the PR.
The CI does not include any Windows tests; but we can add that suite later.

[neptoess]

Hey @neptoess Thanks for continuing to update this PR.
I've fixed up the test so I could check for any regressions in the PR.
The CI does not include any Windows tests; but we can add that suite later.

I won't sign up to do it here, but I did this with Travis CI for confluent-kafka-go and it wasn't terrible
https://github.com/confluentinc/confluent-kafka-go/pull/564/files

[neptoess]

I think I fixed it

[rogchap]

All green. Looking good. Thanks again @neptoess

[rogchap]

Published as part of 0.3.0