pigeonhole: Fix managesieve-login needs libdovecot-login

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

This patch modifed dovecot-config that gets installed on the assumption
that users of libdovecot will also be cross-compiled (a safe bet).
Index: dovecot-2.3.2.1/Makefile.am
===================================================================
--- dovecot-2.3.2.1.orig/Makefile.am
+++ dovecot-2.3.2.1/Makefile.am
@@ -73,7 +73,7 @@ install-exec-hook:
 	grep -v '^LIBDOVECOT_.*_INCLUDE' dovecot-config | \
 	grep -v '^LIBDOVECOT.*_DEPS' | sed \
 	-e "s|^\(DOVECOT_INSTALLED\)=.*$$|\1=yes|" \
-	-e "s|^\(LIBDOVECOT\)=.*$$|\1='-L$(pkglibdir) -ldovecot'|" \
+	-e "s|^\(LIBDOVECOT\)=.*$$|\1='-L$(STAGING_DIR)$(pkglibdir) -ldovecot'|" \
 	-e "s|^\(LIBDOVECOT_LOGIN\)=.*$$|\1='-ldovecot-login $(SSL_LIBS)'|" \
 	-e "s|^\(LIBDOVECOT_SQL\)=.*$$|\1=-ldovecot-sql|" \
 	-e "s|^\(LIBDOVECOT_COMPRESS\)=.*$$|\1=-ldovecot-compression|" \
@@ -81,7 +81,8 @@ install-exec-hook:
 	-e "s|^\(LIBDOVECOT_LDA\)=.*$$|\1=-ldovecot-lda|" \
 	-e "s|^\(LIBDOVECOT_LIBFTS\)=.*$$|\1=-ldovecot-fts|" \
 	-e "s|^\(LIBDOVECOT_STORAGE\)=.*$$|\1='-ldovecot-storage $(LINKED_STORAGE_LDADD)'|" \
-	-e "s|^\(LIBDOVECOT_INCLUDE\)=.*$$|\1=-I$(pkgincludedir)|" \
+	-e "s|^\(LIBDOVECOT_INCLUDE\)=.*$$|\1=-I$(STAGING_DIR)$(pkgincludedir)|" \
+	-e "s|^\(DOVECOT_BINARY_LDFLAGS=\".*\)\"$$|\1 -Wl,-rpath -Wl,$(pkglibdir)\"|" \
 	> $(DESTDIR)$(pkglibdir)/dovecot-config
 
 uninstall-hook: