Log Message:
Update to 0.9.15.1. Changes:
- Fix build error on system that use native awk implementation in place of GNU awk
  (Pierre Chifflier <chifflier at inl.fr>), fix #256.
- Avoid a prelude-string fatal assertion, by denying copy/cloning of an empty
  prelude-string.
- Correction to the 'prelude-admin send' help message.
- Convert prelude-string to use prelude_return_if_fail() in place of prelude_log().