Extract rbconfig patch to rbconfig-patch.rb

This way, the new values are available without a full `require 'mkmf'`.
Note that `rbconfig.rb` would hide the original `rbconfig` from the `$LOAD_PATH` which the current solution depends on to “require super”.

README.md

@@ -4,17 +4,18 @@ However, [__Minimum__ GNU for Windows (MinGW)](https://github.com/niXman/mingw-b
 a key component of MSYS2, is sufficient for half of the jobs all on its own.
 
 `mingw-make` is a set of tiny non-intrusive mods for [RubyInstaller2 for Windows](https://rubyinstaller.org)
-that solves the hurdles on the other half minimally.
+that solves the hurdles on the other half minimally, namely by replacing `rbconfig` entries of compilation commands.
 With the aid of the secret [`ruby -run`](https://github.com/ruby/un) capability it has unleashed,
 all it took was a couple of quick hacks and we have ourselves a lightweight Devkit.
 
 Setups with MinGW and RubyInstaller’s prebuilds with these patches are ideal to keep footprints small,
-for experts strongly recommend WSL for a fully-fledged Ruby environment.
+though experts strongly recommend WSL for a fully-fledged Ruby environment.
 
 ### Compatibility
 
 This is a newly published experiment; I have only tested with the following projects.
-In theory, it’s compatible with anything that doesn’t leave [`mkmf`](https://rubyapi.org/o/MakeMakefile)’s comfort zone,
+In theory, it’s compatible with anything that doesn’t leave
+[`rbconfig`](https://rubyapi.org/o/RbConfig)+[`mkmf`](https://rubyapi.org/o/MakeMakefile)’s comfort zone,
 which I expect to be the majority of C-based gems. (It *will indeed* “take a while” when “Building native extensions”.)
 Please do [let me know](https://github.com/ParadoxV5/ruby-mingw-make/issues)
 if it doesn’t meet the expectations on something not unusual.
@@ -74,10 +75,15 @@ end
 ```
 
 ### Use manually
-Specify `mingw32-make` as the `MAKE` tool and prepend the `lib/mingw-make/mkmf.rb` script to the Ruby `$LOAD_PATH`.
+*Prepend* (so it takes precedence) the [`lib/mingw-make/mkmf.rb`](lib/mingw-make/mkmf.rb)
+script to the Ruby `$LOAD_PATH`. Because RubyGems `require 'rbconfig'`,
+we must explicitly apply [the `rbconfig` replacements](lib/mingw-make/rbconfig-patch.rb).
+```bat
+set RUBYOPT=-Ipath/to/gems/mingw-make/lib/mingw-make/ -rrbconfig-patch
+```
+You may also need to specify a fallback for the `MAKE` tool, e.g.:
 ```bat
 set MAKE=mingw32-make
-set RUBYOPT=-Ipath/to/gems/mingw-make/lib/mingw-make/
 ```
 
 

lib/mingw-make.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
-# Set up `RUBYOPT=-I/absolute/path/to/lib/mingw-make/` for subprocesses
-ENV['RUBYOPT'] = "#{ENV['RUBYOPT']} -I#{File.join __dir__, 'mingw-make'}"
+# Set up `RUBYOPT=-I/absolute/path/to/lib/mingw-make/ -rrbconfig-patch` for subprocesses
+ENV['RUBYOPT'] = "#{ENV['RUBYOPT']} -I#{File.join __dir__, 'mingw-make'} -rrbconfig-patch"
 ENV['MAKE'] ||= 'make'

lib/mingw-make/mkmf.rb

@@ -1,24 +1,5 @@
 # frozen_string_literal: true
-
-require 'rbconfig'
-
-# Use `ruby -run` fall-backs 
-   install   =  'ruby -run -e install -- -p'
-fixed = {
-  'RM'       => 'ruby -run -e rm -- -f',
-  'RMALL'    => 'ruby -run -e rm -- -rf',
-  'RMDIRS'   => 'ruby -run -e rmdir -- -p',
-  'MAKEDIRS' => 'ruby -run -e mkdir -- -p',
-  'CP'       => 'ruby -run -e cp --',
-  'INSTALL'  => install
-}
-install_args = {
-  'INSTALL_PROG' => '-m 0755',
-  'INSTALL_DATA' => '-m 0644'
-}
-RbConfig::MAKEFILE_CONFIG.merge! fixed, install_args.transform_values { "$(INSTALL) #{_1}" }
-RbConfig::CONFIG.merge!          fixed, install_args.transform_values { "#{install} #{_1}" }
-
+require_relative 'rbconfig-patch'
 require File.join RbConfig::CONFIG['rubylibdir'], 'mkmf'
 
 module MakeMakefile

lib/mingw-make/rbconfig-patch.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+# Normally, RubyGems `require`s `rbconfig` itself, so overloading `require 'rbconfig'` may be ineffective.
+# Howëver, if RubyGems are disabled (`--disable-gems`), nameing this file `rbconfig.rb` would conflict with the design,
+# which depends on the original {RbConfig} to bypass the modified `$LOAD_PATH` to fetch the original {MakeMakefile}.
+
+require 'rbconfig'
+
+# Use `ruby -run` fall-backs 
+install      =  'ruby -run -e install -- -p'
+fixed = {
+  'RM'       => 'ruby -run -e rm -- -f',
+  'RMALL'    => 'ruby -run -e rm -- -rf',
+  'RMDIRS'   => 'ruby -run -e rmdir -- -p',
+  'MAKEDIRS' => 'ruby -run -e mkdir -- -p',
+  'CP'       => 'ruby -run -e cp --',
+  'INSTALL'  => install
+}
+install_args = {
+  'INSTALL_PROG' => '-m 0755',
+  'INSTALL_DATA' => '-m 0644'
+}
+RbConfig::MAKEFILE_CONFIG.merge! fixed, install_args.transform_values { "$(INSTALL) #{_1}" }
+RbConfig::CONFIG.merge!          fixed, install_args.transform_values { "#{install} #{_1}" }