message: configure: fix the diff when run for the first time
  Currently we have display an (ignored) error because we diff with a
  non-existing file.
  And while at it, make the created `drake` executable.
diff: @@ -4,6 +4,7 @@ import os
  import os.path
  import shutil
  import subprocess
  +import stat
  import sys
  import platform
  @@ -175,5 +176,7 @@ with open('%s/.drake.tmp' % options.build_dir, 'w') as drake:
  build_dir = options.build_dir
  tmp = '%s/.drake.tmp' % build_dir
  drake = '%s/drake' % build_dir
  -diff = subprocess.call(['diff', '-u', drake, tmp])
  +subprocess.check_call(['diff', '-u', drake if os.path.isfile(drake) else '/dev/null', tmp])
  shutil.move(tmp, drake)
  +# chmod +x
  +os.chmod(drake, os.stat(drake).st_mode | stat.S_IEXEC)
  