For the macOS -bundle_loader linker option, we need a path to the
Ruby exectuable. $(RUBY) is not necessarily a path since it could
be a command line invocation. That happens during build with
runruby.rb and can happen post installation if the user passes
the --ruby option to a extconf.rb. Use $(bindir) to locate
the executable instead.

Before installation, $(bindir) doesn't exist, so we need to be
able to override $(BUILTRUBY) in such situations so test-spec
and bundled extensions could build. Use a new mkmf global,
$builtruby, to do this; set it in fake.rb and in extmk.rb.

Our build system is quite complex...