mkconfig.rb: clear exec_prefix

* tool/mkconfig.rb: clear exec_prefix which may differ from prefix,
  before expanding rubyarchdir to remove prefix.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39297 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/mkconfig.rb

@@ -167,7 +167,8 @@ def vars.expand(val, config = self)
   val
 end
 vars["prefix"] = ""
-prefix = vars.expand(vars["rubyarchdir"]).sub!(vars.expand(vars["exec_prefix"]), '')
+vars["exec_prefix"] = ""
+prefix = vars.expand(vars["rubyarchdir"])
 print "  TOPDIR = File.dirname(__FILE__).chomp!(#{prefix.dump})\n"
 print "  DESTDIR = ", (drive ? "TOPDIR && TOPDIR[/\\A[a-z]:/i] || " : ""), "'' unless defined? DESTDIR\n"
 print <<'ARCH' if universal