fiddle: fix $(SUBMAKE_PRE)

* ext/fiddle/extconf.rb: fix $(SUBMAKE_PRE) to chdir to
  $(LIBFFI_DIR) instead of $(@D), since $(LIBFFI_A) is not
  underneath libffi but under .libs.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57593 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/fiddle/extconf.rb

@@ -149,9 +149,9 @@ create_makefile 'fiddle' do |conf|
   if !libffi
     next conf << "LIBFFI_CLEAN = none\n"
   elsif $gnumake && !$nmake
-    submake_arg = "-C $(@D)\n"
+    submake_arg = "-C $(LIBFFI_DIR)\n"
   else
-    submake_pre = "cd $(@D) && #{config_string("exec")}".strip
+    submake_pre = "cd $(LIBFFI_DIR) && #{config_string("exec")}".strip
   end
   if $nmake
     cmd = "$(RUBY) -C $(LIBFFI_DIR) #{libffi_config} --srcdir=$(LIBFFI_SRCDIR)"