Makefile: Prefer Ruby 2.7 if available

[liskin]

Ruby 3.0, which is now default in Debian (and likely many other distros), isn't officially supported by the github/pages-gem and some of its dependencies (despite patches being available for over a year—WTF), so make serve doesn't work.

As a workaround, prefer Ruby 2.7 (the "bundle2.7" command) if available.

Related: #37

[liskin]

Not sure if the naming of bundle2.7 is specific to Debian or something we can expect to be available elsewhere. Can someone check on your distros pls?

[geekosaur]

I'm on Ubuntu so it's unlikely to help with that.

[slotThe]

Not sure if the naming of bundle2.7 is specific to Debian or something we can expect to be available elsewhere. Can someone check on your distros pls?

I believe it is. Neither Void nor Arch ship ruby 2.7 anymore (and both tell you to install bundler with gem install anyways and don't have a separate package for it). Nix's ruby package is still on 2.7.5 by default, but AFAICT the installed bundler executable is just called that. Though I guess if it helps some people that's already better than the current situation

[liskin]

Oh crap. I wish there was something like Python's -m in Ruby so that we could do something like ruby2.7 -m bundle …. And even that assumes there's a ruby2.7 binary at all. Do you at least have ruby3.0 in your $PATH or does Arch/Void not ship that either?

[slotThe]

Oh, actually I was talking nonesense. Void does not have a ruby2.7 but Arch has—seems like I just missed it. It installs bundle-2.7 and bundler-2.7 as executables.

Do you at least have ruby3.0 in your $PATH or does Arch/Void not ship that either?

At least for Void it seems to be the latter :/

[liskin]

Okay, that's helpful :-)
Updated to handle Arch as well, and added a comment with more info.