diff --git a/array.c b/array.c
index 4a4c44562d28bf..90e83fd3ad972f 100644
--- a/array.c
+++ b/array.c
@@ -3163,7 +3163,7 @@ rb_ary_to_a(VALUE ary)
  *  forms each sub-array into a key-value pair in the new hash:
  *
  *    a = [['foo', 'zero'], ['bar', 'one'], ['baz', 'two']]
- *    a.to_h # => {"foo"=>"zero", "bar"=>"one", "baz"=>"two"}
+ *    a.to_h # => {"foo" => "zero", "bar" => "one", "baz" => "two"}
  *    [].to_h # => {}
  *
  *  With a block given, the block must return a 2-element array;
@@ -3172,7 +3172,7 @@ rb_ary_to_a(VALUE ary)
  *
  *    a = ['foo', :bar, 1, [2, 3], {baz: 4}]
  *    a.to_h {|element| [element, element.class] }
- *    # => {"foo"=>String, :bar=>Symbol, 1=>Integer, [2, 3]=>Array, {:baz=>4}=>Hash}
+ *    # => {"foo" => String, bar: Symbol, 1 => Integer, [2, 3] => Array, {baz: 4} => Hash}
  *
  *  Related: see {Methods for Converting}[rdoc-ref:Array@Methods+for+Converting].
  */
diff --git a/common.mk b/common.mk
index 8891b65ed9b306..5623b50b853cd9 100644
--- a/common.mk
+++ b/common.mk
@@ -938,19 +938,23 @@ rbconfig.rb: $(RBCONFIG)
 
 $(HAVE_BASERUBY:no=)$(RBCONFIG)$(HAVE_BASERUBY:no=): $(PREP)
 $(RBCONFIG): $(tooldir)/mkconfig.rb config.status $(srcdir)/version.h $(srcdir)/common.mk
+$(RBCONFIG): unicode-version
+
+unicode-version:
 	$(Q)$(BOOTSTRAPRUBY) -n \
 	-e 'BEGIN{version=ARGV.shift;mis=ARGV.dup}' \
 	-e 'END{abort "UNICODE version mismatch: #{mis}" unless mis.empty?}' \
 	-e '(mis.delete(ARGF.path); ARGF.close) if /ONIG_UNICODE_VERSION_STRING +"#{Regexp.quote(version)}"/o' \
 	$(UNICODE_VERSION) $(UNICODE_DATA_HEADERS)
+
+$(RBCONFIG):
 	$(Q)$(BOOTSTRAPRUBY) $(tooldir)/mkconfig.rb \
 		-arch=$(arch) -version=$(RUBY_PROGRAM_VERSION) \
 		-install_name=$(RUBY_INSTALL_NAME) \
 		-so_name=$(RUBY_SO_NAME) \
 		-unicode_version=$(UNICODE_VERSION) \
 		-unicode_emoji_version=$(UNICODE_EMOJI_VERSION) \
-	> rbconfig.tmp
-	$(IFCHANGE) "--timestamp=$@" rbconfig.rb rbconfig.tmp
+	| $(IFCHANGE) "--timestamp=$@" rbconfig.rb -
 
 test-rubyspec: test-spec
 yes-test-rubyspec: yes-test-spec
diff --git a/lib/rubygems.rb b/lib/rubygems.rb
index 31abb7e56987ea..2139264629c422 100644
--- a/lib/rubygems.rb
+++ b/lib/rubygems.rb
@@ -1410,9 +1410,7 @@ def default_gem_load_paths
 
 # REFACTOR: This should be pulled out into some kind of hacks file.
 begin
-  ##
   # Defaults the operating system (or packager) wants to provide for RubyGems.
-
   require "rubygems/defaults/operating_system"
 rescue LoadError
   # Ignored
@@ -1427,9 +1425,7 @@ def default_gem_load_paths
 end
 
 begin
-  ##
   # Defaults the Ruby implementation wants to provide for RubyGems
-
   require "rubygems/defaults/#{RUBY_ENGINE}"
 rescue LoadError
 end
diff --git a/spec/ruby/optional/capi/spec_helper.rb b/spec/ruby/optional/capi/spec_helper.rb
index d937c967d062fc..49ce23d87432e3 100644
--- a/spec/ruby/optional/capi/spec_helper.rb
+++ b/spec/ruby/optional/capi/spec_helper.rb
@@ -89,6 +89,12 @@ def compile_extension(name)
           $ruby = ENV.values_at('RUBY_EXE', 'RUBY_FLAGS').join(' ')
           # MRI magic to consider building non-bundled extensions
           $extout = nil
+          if RbConfig::CONFIG.key?("buildlibdir") # The top directory where the libruby is built.
+            # Prepend the dummy macro to bypass the conversion in `with_destdir` on DOSISH
+            # platforms, where the drive letter is replaced with `$(DESTDIR)`.  `DESTDIR` is
+            # overridden by the command line argument bellow.
+            RbConfig::MAKEFILE_CONFIG["buildlibdir"] = "$(empty)" + RbConfig::CONFIG["buildlibdir"]
+          end
           append_cflags '-Wno-declaration-after-statement'
           #{"append_cflags #{ruby_repository_extra_include_dir.inspect}" if ruby_repository_extra_include_dir}
           create_makefile(#{ext.inspect})
@@ -100,7 +106,7 @@ def compile_extension(name)
 
       # Do not capture stderr as we want to show compiler warnings
       make, opts = setup_make
-      output = IO.popen([make, "V=1", "DESTDIR=", opts], &:read)
+      output = IO.popen([*make, "V=1", "DESTDIR=", opts], &:read)
       raise "#{make} failed:\n#{output}" unless $?.success?
       $stderr.puts output if debug
 
@@ -117,22 +123,34 @@ def compile_extension(name)
 def setup_make
   make = ENV['MAKE']
   make ||= (RbConfig::CONFIG['host_os'].include?("mswin") ? "nmake" : "make")
-  make_flags = ENV["MAKEFLAGS"] || ''
+  env = %w[MFLAGS MAKEFLAGS GNUMAKEFLAGS].to_h {|var| [var, ENV[var]]}
+  make_flags = env["MAKEFLAGS"] || ''
 
   # suppress logo of nmake.exe to stderr
   if File.basename(make, ".*").downcase == "nmake" and !make_flags.include?("l")
-    ENV["MAKEFLAGS"] = "l#{make_flags}"
+    env["MAKEFLAGS"] = "l#{make_flags}"
   end
 
   opts = {}
   if /(?:\A|\s)--jobserver-(?:auth|fds)=(\d+),(\d+)/ =~ make_flags
     [$1, $2].each do |fd|
-      fd = fd.to_i(10)
+      fd = IO.for_fd(fd.to_i(10), autoclose: false)
       opts[fd] = fd
+    rescue
+      # Jobserver is not usable, maybe no `+` flag or on Windows.
+      job_options = /\A\s*(?:-j\d+\s*|-\S+\Kj\d+)|(?:\G|\s)\K--jobserver-(?:auth|fds)=\S+\s*/
+      env["MAKEFLAGS"] = make_flags.gsub(job_options, '')
+      %w[GNUMAKEFLAGS MFLAGS].each do |var|
+        if flags = env[var]
+          env[var] = flags.gsub(job_options, '')
+        end
+      end
+      opts.clear
+      break
     end
   end
 
-  [make, opts]
+  [[env, make], opts]
 end
 
 def load_extension(name)