Debian Buster: Configuring Dovecot for FTS with fts-xapian

To start off, we need to install some packages:

apt install build-essential git 
apt install dovecot-dev libicu-dev libxapian-dev libsqlite3-dev

Then let's clone the fts-xapian git repository:

git clone https://github.com/grosjo/fts-xapian
cd fts-xapian

Let's compile...

autoreconf -vi
./configure --with-dovecot=/usr/lib/dovecot
make
sudo make install

"/usr/lib/dovecot" is the base directory of /usr/lib/dovecot/dovecot-config, installed by dovecot-dev. Update your dovecot.conf as described.

And finally, if it's not a fresh install

# Just index all mailboxes of all users
doveadm index -A -q \*
# If you want to know what's going on and what "index" is doing:
doveadm -Dv index -u $DOVECOTUSER \*
# or
doveadm -Dv index -u $DOVECOTUSER INBOX