Build the extension library only when CRuby (Fix #69)

On JRuby no C compiler is available and this call results in a runtime
error.

Rakefile

@@ -1,13 +1,15 @@
 require "bundler/gem_tasks"
 require "rake/testtask"
-require "ruby-core/extensiontask"
 
-helper = Bundler::GemHelper.instance
-extask = RubyCore::ExtensionTask.new(helper.gemspec)
-task :test => :compile
+if RUBY_ENGINE == "ruby"
+  require "ruby-core/extensiontask"
+  helper = Bundler::GemHelper.instance
+  extask = RubyCore::ExtensionTask.new(helper.gemspec)
+  task :test => :compile
+end
 
 Rake::TestTask.new(:test) do |t|
-  t.libs.unshift(*extask.libs)
+  t.libs.unshift(*extask.libs) if extask
   t.libs << "test/lib"
   t.ruby_opts << "-rhelper"
   t.test_files = FileList["test/**/test_*.rb"]

ext/win32/resolv/extconf.rb

@@ -1,5 +1,5 @@
 require 'mkmf'
-if have_library('iphlpapi', 'GetNetworkParams')
+if RUBY_ENGINE == "ruby" and have_library('iphlpapi', 'GetNetworkParams')
   create_makefile('win32/resolv')
 else
   File.write('Makefile', "all clean install:\n\t@echo Done: $(@)\n")