Set SOURCE_DATE_EPOCH to make builds reproducible

Author: nobu

Rakefile

@@ -6,3 +6,9 @@ Rake::TestTask.new(:test) do |t|
 end
 
 task :default => :test
+
+task "build" => "date_epoch"
+
+task "date_epoch" do
+  ENV["SOURCE_DATE_EPOCH"] = IO.popen(%W[git -C #{__dir__} log -1 --format=%ct], &:read).chomp
+end