nmake didn't support -j flag

Author: hsbt

lib/rubygems/ext/ext_conf_builder.rb

@@ -40,7 +40,9 @@ def self.build(extension, dest_path, results, args = [], lib_dir = nil, extensio
       end
 
       ENV["DESTDIR"] = nil
-      ENV["MAKEFLAGS"] ||= "-j#{Etc.nprocessors + 1}"
+      unless RbConfig::CONFIG["MAKE"] =~ /nmake/
+        ENV["MAKEFLAGS"] ||= "-j#{Etc.nprocessors + 1}"
+      end
 
       make dest_path, results, extension_dir, tmp_dest_relative, target_rbconfig: target_rbconfig