When trying to use managesieve my MUA complained sieve wasn't supported.
On investigation dovecot logs the following could be seen:

    Aug 16 00:28:44 managesieve-login: Fatal: master:
      service(managesieve-login): child 1430 returned error 127
    Aug 16 00:31:32 managesieve-login: Error: Error loading shared
      library libdovecot-login.so.0: No such file or directory
      (needed by /usr/lib/dovecot/managesieve-login)
    Aug 16 00:31:32 managesieve-login: Error: Error loading
      shared library libdovecot.so.0: No such file or directory
      (needed by /usr/lib/dovecot/managesieve-login)
    Aug 16 00:31:32 managesieve-login: Error: Error relocating
      /usr/lib/dovecot/managesieve-login: net_ip2addr: symbol not found

The issue (verified with readelf on non-working build and build with my fix)
is that there is no RPATH information in the pigeonhole binaries (like
managesieve-login).

The dovecot-config that is 'installed' in the staging dir
assumes that plugins will be built on the same host as the installed files.
The 'installed' dovecot-config (partial) looks like:

LIBDOVECOT='-L/usr/lib/dovecot -ldovecot'
LIBDOVECOT_LOGIN='-ldovecot-login -L/home/user/Build/openwrt/openwrt-ath79/staging_dir/target-mips_24kc_musl/usr/lib -lssl -lcrypto'
LIBDOVECOT_SQL=-ldovecot-sql
LIBDOVECOT_COMPRESS=-ldovecot-compression
LIBDOVECOT_LDA=-ldovecot-lda
LIBDOVECOT_STORAGE='-ldovecot-storage '
LIBDOVECOT_DSYNC=-ldovecot-dsync
LIBDOVECOT_LIBFTS=-ldovecot-fts

The solution I used was to modify dovecot-config to add the rpath to
DOVECOT_BINARY_LDFLAGS, and to replace references to host directories
with staging_dir directories.

This results in RPATH being added as it needs to be, and results in a working
version of dovecot+pigeonhole.

NB: I've also touched package version of pigeonhole as this change in dovecot
results in (needed) binary changes for pigeonhole.

Signed-off-by: Daniel F. Dickinson <cshored@thecshore.com>