Index: mkconfig.rb
===================================================================
--- mkconfig.rb.orig	2008-11-21 19:55:28.000000000 +0100
+++ mkconfig.rb	2008-11-21 20:34:41.653180779 +0100
@@ -22,6 +22,7 @@ def (config = "").write(arg)
 end
 $stdout = config
 
+configured_prefix = ""
 fast = {'prefix'=>TRUE, 'ruby_install_name'=>TRUE, 'INSTALL'=>TRUE, 'EXEEXT'=>TRUE}
 print %[
 # This file was created by #{mkconfig} when ruby was built.  Any
@@ -86,6 +87,7 @@ File.foreach "config.status" do |line|
     val.gsub!(/ +(?!-)/, "=") if name == "configure_args" && /mswin32/ =~ RUBY_PLATFORM
     val = val.gsub(/\$(?:\$|\{?(\w+)\}?)/) {$1 ? "$(#{$1})" : $&}.dump
     if /^prefix$/ =~ name
+      configured_prefix = val
       val = "(TOPDIR || DESTDIR + #{val})"
     end
     v = "  CONFIG[\"#{name}\"] #{vars[name] ? '<< "\n"' : '='} #{val}\n"
@@ -103,7 +105,7 @@ end
 drive = File::PATH_SEPARATOR == ';'
 
 prefix = '/lib/ruby/' + RUBY_VERSION.sub(/\.\d+$/, '') + '/' + RUBY_PLATFORM
-print "  TOPDIR = File.dirname(__FILE__).chomp!(#{prefix.dump})\n"
+print "  TOPDIR = #{configured_prefix}\n"
 print "  DESTDIR = ", (drive ? "TOPDIR && TOPDIR[/\\A[a-z]:/i] || " : ""), "'' unless defined? DESTDIR\n"
 print "  CONFIG = {}\n"
 print "  CONFIG[\"DESTDIR\"] = DESTDIR\n"