[ruby/resolv] Add spec extensions

https://github.com/ruby/resolv/commit/3189d16b69

ext/win32/resolv/extconf.rb

@@ -1,3 +1,6 @@
+require 'mkmf'
 if have_library('iphlpapi', 'GetNetworkParams')
   create_makefile('win32/resolv')
+else
+  File.write('Makefile', "all clean install:\n\t@echo Done: $(@)\n")
 end

lib/resolv.gemspec

@@ -16,6 +16,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = "https://github.com/ruby/resolv"
   spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
   spec.licenses      = ["Ruby", "BSD-2-Clause"]
+  spec.extensions << "ext/win32/resolv/extconf.rb"
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage