Add support for TokenMecab on Alpine Linux

Currently, Alpine Linux images don't support TokenMecab.

$ docker pull groonga/pgroonga:latest-alpine-14
docker run -p 5432:5432 --env POSTGRES_HOST_AUTH_METHOD=trust --env POSTGRES_DB=PGroonga --env POSTGRES_PASSWORD=PGroonga --env POSTGRES_USER=PGroonga groonga/pgroonga:latest-alpine-14

$ psql -h localhost -p 5432 -U PGroonga

PGroonga=# CREATE EXTENSION pgroonga;
CREATE EXTENSION
PGroonga=# CREATE TABLE memos (
  id integer,
  content text
);
CREATE TABLE
PGroonga=# CREATE INDEX pgroonga_content_index ON memos
  USING pgroonga (content)
  WITH (tokenizer='TokenMecab');
ERROR:  pgroonga: [option][tokenizer][validate] invalid tokenizer: <TokenMecab>: [info][set][default-tokenizer][(anonymous)] unknown tokenizer: <TokenMecab>

pgroonga / docker

Add support for TokenMecab on Alpine Linux #16