/usr/lib/ruby/vendor_ruby/bundler/templates/newgem/ext/newgem/extconf.rb.tt is in ruby-bundler 1.16.1-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | require "mkmf"
create_makefile(<%= config[:makefile_path].inspect %>)
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