Remove mkmf as a dependency

[botandrose]

Requiring mkmf has some unfortunate side-effects, like polluting the top-level namespace causing naming collisions with e.g. ffi gem (see pbhogan/scrypt#47). In fact, it turns out that Ruby core recommends only requiring mkmf in extconf.rb, and not anywhere else (see https://bugs.ruby-lang.org/issues/12370#note-4).

Therefore, this PR removes the mkmf dependency, and relies on Open3.popenx methods to be able to find the executables themselves via the system PATH. This also simplifies things a bit, removing the need for the elm_executable variable.

What do you think?

[fbonetti]

I was wondering why you removed mkmf in your last PR but this makes perfect sense now. Thanks for opening this. I'll release this as a patch.

[botandrose]

🎉

[fbonetti]

@botandrose https://rubygems.org/gems/elm-compiler/versions/0.2.1