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

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

Author: nobu

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")