山本です。

>そのため、ext/extmk.rb に手を入れてしまいましたが、こんな感じでいいでしょうか？
>HEAD も ruby_1_8 も動いているようです。

こちらを少し訂正します。元が force delete になっているのでそれを踏襲して・・・

Index: ext/extmk.rb
===================================================================
RCS file: /src/ruby/ext/extmk.rb,v
retrieving revision 1.75
diff -u -w -b -p -r1.75 extmk.rb
--- ext/extmk.rb	8 Apr 2005 11:18:01 -0000	1.75
+++ ext/extmk.rb	12 Apr 2005 15:20:24 -0000
@@ -363,6 +363,7 @@ if $extout
   if $install
     Config.expand(dest = "#{$destdir}#{$rubylibdir}")
     FileUtils.cp_r(extout+"/.", dest, :verbose => true, :noop => $dryrun)
+    FileUtils.rm_f(Dir[File.join(dest,"**","*.tds")]) if $bccwin
     exit
   end
   unless $ignore

Index: bcc32/Makefile.sub
===================================================================
RCS file: /src/ruby/bcc32/Makefile.sub,v
retrieving revision 1.71
diff -u -w -b -p -r1.71 Makefile.sub
--- bcc32/Makefile.sub	3 Mar 2005 09:44:32 -0000	1.71
+++ bcc32/Makefile.sub	12 Apr 2005 14:40:27 -0000
@@ -426,10 +426,6 @@ $(RUBY_INSTALL_NAME).rc $(RUBYW_INSTALL_
 			-so_name=$(RUBY_SO_NAME) \
 			. $(icondirs) $(srcdir)win32
 
-post-install-ext::
-	$(MINIRUBY) -I$(srcdir)lib -rrbconfig -rfileutils \
-		-e 'FileUtils.rm_f(Dir[File.join(Config::CONFIG["archdir"],"**","*.tds")])'
-
 clean-local::
 		@$(RM) ext\extinit.c ext\extinit.$(OBJEXT) *.tds *.il? $(RUBY_SO_NAME).lib
 		@$(RM) $(RUBY_INSTALL_NAME).res $(RUBYW_INSTALL_NAME).res $(RUBY_SO_NAME).res