Replace detect-libc with isAlpine check?

[vweevers]

Same as in node-gyp-build, we can do:

var platform = os.platform()
var libc = process.env.LIBC || (isAlpine(platform) ? 'musl' : 'glibc')

function isAlpine (platform) {
  return platform === 'linux' && fs.existsSync('/etc/alpine-release')
}

This is much more lightweight, see prebuild/node-gyp-build#14 (comment).

@lovell (as the author of #55) any objections?

[vweevers]

While we're at it, let's also read env.npm_config_libc to support npm i --libc=musl. That use case makes more sense than what we currently support (prebuild-install --libc=musl).

[lovell]

I think the desire is to correctly detect musl libc, of which Alpine Linux is just one consumer.

An increasing number of Linux flavours also use musl libc but won't have /etc/alpine-release, e.g. Void, OpenWrt.

The existsSync check proposed here would make a good addition to detect-libc and could occur before spawning any processes if that's a problem. Happy to accept a PR.

[vweevers]

The existsSync check proposed here would make a good addition to detect-libc and could occur before spawning any processes if that's a problem

Nah, it would still spawn processes if the platform is not Alpine, which is more common. And in the context of detect-libc it's probably more important to be correct (e.g. older Alpine does use glibc).

An increasing number of Linux flavours also use musl libc but won't have /etc/alpine-release, e.g. Void, OpenWrt.

Roger.