Fix large project generated script

Rakefile.rb

@@ -76,7 +76,7 @@ task :gen_large_project, [:size] => :build_dist do |task, args|
   end
   File.open("large_project/Rsconscript", "w") do |fh|
     fh.puts <<EOF
-build do
+default do
   Environment.new do |env|
     env.Program("project", glob("src/*.c"))
   end