Use Github Actions to build J2V8 for macOS and Windows

[jonek]

Github Actions could potentially be used to build J2V8 for the macOS and Windows target platforms.

This is a description of the available build environments: https://help.github.com/en/articles/virtual-environments-for-github-actions#supported-virtual-environments-and-hardware-resources

Branch v8-7.4.288-build contains the basic setup to build for the Linux and Android target platforms.
This could serve as a starting point.

[jonek]

@Cretezy please share your efforts mentioned in #477 (comment) regarding the macOS and Windows platforms on this issue.

[Cretezy]

Although I still can't build, and having issues with my fork's Actions for some reason (never starting). I have the Windows/MacOS build code at https://github.com/Cretezy/J2V8/blob/v8-7.4.288-build/.github/workflows/main.yml, but can't test it

My question is if cross-compiling is possible from a Linux environment. If not, we could use Windows/Mac environments for building

Update: Actions finally worked. Will start working on Windows/MacOS build

[Cretezy]

Findings:

• Linux works as expected
• Windows doesn't work with Docker. It seems like it's a symlink issue, not sure why: unable to prepare context: unable to evaluate symlinks in Dockerfile path
• Mac doesn't have Docker included.

Vagrant builds seemed to be fully broken since it doesn't implement the exec_v8_build function

Update:

• Windows: Making progressed and fixed the Docker issue, now hitting some new Docker issues
• Mac: Had to fix some CMake issues to try to build without Docker, still doesn't seem to be working as it can't find JNI headers