Fixup with mswin and nmake build for -j flag

Author: hsbt

lib/rubygems/ext/ext_conf_builder.rb

@@ -40,7 +40,7 @@ def self.build(extension, dest_path, results, args = [], lib_dir = nil, extensio
       end
 
       ENV["DESTDIR"] = nil
-      unless RbConfig::CONFIG["MAKE"]&.include?("nmake")
+      unless RUBY_PLATFORM =~ /mswin/ && RbConfig::CONFIG["configure_args"]&.include?("nmake")
         ENV["MAKEFLAGS"] ||= "-j#{Etc.nprocessors + 1}"
       end