[ruby/io-wait] Make the gem a noop on Rubies older than 2.6

Ref: https://github.com/mikel/mail/pull/1439

Some gems depend on io-wait, but still support older rubies,
so they have to chose between droping support or not listing io-wait.

But io-wait could act a a noop on older rubies.

75fcb74c32

ext/io/wait/extconf.rb

@@ -1,9 +1,12 @@
 # frozen_string_literal: false
 require 'mkmf'
-target = "io/wait"
 
-have_func("rb_io_wait")
-unless macro_defined?("DOSISH", "#include <ruby.h>")
+if RUBY_VERSION < "2.6"
+  File.write("Makefile", dummy_makefile($srcdir).join(""))
+else
+  target = "io/wait"
+  have_func("rb_io_wait")
+  unless macro_defined?("DOSISH", "#include <ruby.h>")
     have_header(ioctl_h = "sys/ioctl.h") or ioctl_h = nil
     fionread = %w[sys/ioctl.h sys/filio.h sys/socket.h].find do |h|
       have_macro("FIONREAD", [h, ioctl_h].compact)
@@ -12,9 +15,10 @@ unless macro_defined?("DOSISH", "#include <ruby.h>")
       $defs << "-DFIONREAD_HEADER=\"<#{fionread}>\""
       create_makefile(target)
     end
-else
+  else
     if have_func("rb_w32_ioctlsocket", "ruby.h")
       have_func("rb_w32_is_socket", "ruby.h")
       create_makefile(target)
     end
+  end
 end

ext/io/wait/io-wait.gemspec

@@ -10,7 +10,6 @@ Gem::Specification.new do |spec|
   spec.description   = %q{Waits until IO is readable or writable without blocking.}
   spec.homepage      = "https://github.com/ruby/io-wait"
   spec.licenses      = ["Ruby", "BSD-2-Clause"]
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.6.0")
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage