[ruby/rubygems] Add MAKEFLAGS=-j by default before compiling:

- Depending on the native extension, it can greatly reduce compilation
  time when executing recipes simultaneously.

  For example on Prism:

  ```
  # Before

  time gem install prism
  Building native extensions. This could take a while...
  Successfully installed prism-1.6.0
  1 gem installed

  gem install prism  3.61s user 0.80s system 95% cpu 4.595 total
  ```

  ```
  # After

  time MAKEFLAGS="-j" gem install prism
  Building native extensions. This could take a while...
  Successfully installed prism-1.6.0
  1 gem installed

  MAKEFLAGS="-j" gem install prism  4.47s user 1.27s system 246% cpu 2.330 total
  ```

  I don't think adding `-j` as a default is harmful, but I'm admitedly
  not very knowledgable when it comes to compiler.

ruby/rubygems@61340081c6

Co-authored-by: Aaron Patterson <[email protected]>

Author: 2 people

lib/rubygems/ext/ext_conf_builder.rb

@@ -40,6 +40,7 @@ def self.build(extension, dest_path, results, args = [], lib_dir = nil, extensio
       end
 
       ENV["DESTDIR"] = nil
+      ENV["MAKEFLAGS"] ||= "-j#{Etc.nprocessors + 1}"
 
       make dest_path, results, extension_dir, tmp_dest_relative, target_rbconfig: target_rbconfig