Incorporated sieve_duplicate plugin into main Pigeonhole tree as a normal extension (vnd.dovecot.duplicate).
Also restructured doc dir a bit in the process.

Made vnd.dovecot.debug extension available to the LDA plugin instead of only the command line tools.

Explicitly set AC_CONFIG_AUX_DIR to prevent issues when compiling Pigeonhole in a sub-directory of the Dovecot tree.

Let configure automatically infer path to installed Dovecot and don't rely on installed libtool .la files anymore (patch by Timo Sirainen).