Fixes #11666: Cleanup _build and pkg directories between runs

Successive runs of the rake task that generates package tarballs
did not remove or overwrite the previous builds. In a local or CI
environment, the tarball built could unknowingly result in no
updates and a stale package.

Rakefile

@@ -1,3 +1,5 @@
+require 'rake/clean'
+
 BUILDDIR = File.expand_path(ENV['BUILDDIR'] || '_build')
 PKGDIR = ENV['PKGDIR'] || File.expand_path('pkg')
 
@@ -19,7 +21,7 @@ end
 
 namespace :pkg do
   desc 'Generate package source tar.bz2'
-  task :generate_source => [PKGDIR, "#{BUILDDIR}/modules"] do
+  task :generate_source => [:clean, PKGDIR, "#{BUILDDIR}/modules"] do
 
     version = File.read('VERSION').chomp
     raise "can't read VERSION" if version.length == 0
@@ -34,4 +36,6 @@ namespace :pkg do
   end
 end
 
+CLEAN.include(BUILDDIR, PKGDIR)
+
 task :default => ['pkg:generate_source']