[ruby-dev:31682] mkmf.rb:310:in `method_missing': undefined method `quote' for nil:NilClass

現在のHEADで、extconf.rbを実行しようとすると
subjectのようなエラーになります。

$ ruby1.9 extconf.rb
checking for termios.h... /usr/lib/ruby/1.9/mkmf.rb:310:in `method_missing': undefined method `quote' for nil:NilClass (NoMethodError)
        from /usr/lib/ruby/1.9/mkmf.rb:310:in `cpp_command'
        from /usr/lib/ruby/1.9/mkmf.rb:343:in `try_cpp'
        from /usr/lib/ruby/1.9/mkmf.rb:711:in `block in have_header'
        from /usr/lib/ruby/1.9/mkmf.rb:572:in `block in checking_for'
        from /usr/lib/ruby/1.9/mkmf.rb:218:in `block (2 levels) in postpone'
        from /usr/lib/ruby/1.9/mkmf.rb:191:in `open'
        from /usr/lib/ruby/1.9/mkmf.rb:218:in `block in postpone'
        from /usr/lib/ruby/1.9/mkmf.rb:191:in `open'
        from /usr/lib/ruby/1.9/mkmf.rb:214:in `postpone'
        from /usr/lib/ruby/1.9/mkmf.rb:571:in `checking_for'
        from /usr/lib/ruby/1.9/mkmf.rb:710:in `have_header'
        from extconf.rb:3:in `<main>'

cpp_commandほか、いくつかのメソッドで
$top_srcdirを参照しているためみたいです。