fake.rb: fix builddir

* tool/fake.rb: builddir should be "." if it is the current
  working directory.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51765 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/fake.rb

@@ -42,6 +42,7 @@ prehook = proc do |extmk|
       pwd.shift
     end
     builddir = File.join([".."]*pwd.size + dir)
+    builddir = "." if builddir.empty?
   end
   $topdir ||= builddir
   $top_srcdir ||= File.join($topdir, srcdir)