Skip build extensions again on mswin and mingw

tool/rbinstall.rb

@@ -853,6 +853,9 @@ module RbInstall
     def write_cache_file
     end
 
+    def build_extensions
+    end if /mswin|mingw/ =~ RUBY_PLATFORM
+
     def shebang(bin_file_name)
       path = File.join(gem_dir, spec.bindir, bin_file_name)
       first_line = File.open(path, "rb") {|file| file.gets}