FreeBSD and Alpine Linux support

[asterite]

It would be really nice if we could automatically generate binaries (tar-gzipped) for these platforms and make them available in the release page after each release. In fact, given that we already have compiled binaries for 0.15.0 and 0.16.0 in these platforms, we could add these binaries right after implementing this issue.

I usually modify tins bits of this project before each release, I never added a new platform before. I know @waj could do this, but someone else should be able to do it too, I guess it's just about changing the Vagrant file.

/cc @ysbaddaden

[ysbaddaden]

I don't know much about omnibus, otherwise I would gladly help.

Notes:

• I added a freebsd VM to Crystal's Vagrantfile (it uses NFS because VirtualBox extensions aren't available);
• I build the FreeBSD tarball in the above VM using a 10 line shell script (make, cp, tar);
• BOEHM GC (and certainly other libs), must be built with per-platform specific compile options + patches; I always patch & recompile the official packages, otherwise libgc won't compile or segfaults.

I don't mind building the Alpine packages. They're built using the official abuild tool and I automated the build so all I have to do is change the version in APKBUILD then run make.

[asterite]

Well, if you want to take care of building packages for these platforms after each release (about once a month) then it's fine with me :-)

Feel free to upload the tar.gz for them in the releases page. I don't know if you have access to that page, though, let me know.

[ysbaddaden]

I uploaded the FreeBSD tarball!