mkmf: Add a way to provide a custom rbconfig.rb file

lib/mkmf.rb

@@ -44,6 +44,17 @@ def quote
 # correctly compile and link the C extension to Ruby and a third-party
 # library.
 module MakeMakefile
+
+  if found_index = ARGV.find_index {|arg| arg.start_with?("--target-rbconfig=") }
+    found = ARGV.delete_at(found_index)
+    target_rbconfig = found.sub(/\A--target-rbconfig=/, "")
+    Kernel.load target_rbconfig, self
+  else
+    ##
+    # The RbConfig for the target platform where the built extension runs.
+    RbConfig = ::RbConfig
+  end
+
   #### defer until this module become global-state free.
   # def self.extended(obj)
   #   obj.init_mkmf