message: farm: make it easier to rename directories in the generated file
  And avoid gratuitous name changes.
diff: @@ -56,8 +56,11 @@ import os
  import os.path
  import sys
  +source_dir = %(source_dir)r
  +install_dir = %(install_dir)r
  +
  # Add drake in the Python path.
  -sys.path.insert(0, os.path.realpath(%(dir_source)r '/drake/src'))
  +sys.path.insert(0, os.path.realpath(source_dir + '/drake/src'))
  import drake
  import drake.cxx
  @@ -103,7 +106,7 @@ if %(project)r == 'Filesystem':
  extra_args['go_toolkit'] = go_toolkit
  extra_args['go_config'] = go_config
  -with drake.Drake(%(dir_source)r) as d:
  +with drake.Drake(source_dir) as d:
  python = %(python)r
  import drake.valgrind
  valgrind = %(valgrind)r
  @@ -116,7 +119,7 @@ with drake.Drake(%(dir_source)r) as d:
  valgrind = valgrind,
  valgrind_tests = valgrind_tests,
  production_build = %(production_build)r,
  - prefix = %(dir_install)r,
  + prefix = install_dir,
  codesign = codesign,
  **extra_args)
  @@ -179,8 +182,8 @@ else:
  raise Exception('invalid OS: %s' % options.os)
  with open('%s/.drake.tmp' % options.build_dir, 'w') as drake:
  - parameters['dir_source'] = os.path.realpath(options.source_dir)
  - parameters['dir_install'] = os.path.realpath(options.install_dir)
  + parameters['source_dir'] = os.path.realpath(options.source_dir)
  + parameters['install_dir'] = os.path.realpath(options.install_dir)
  content = template % parameters
  print(content, file = drake)
  