feat(database): add optional build support for SQLite

Miniflux can be build with `go build -tags=sqlite` to test this. Note that
while it builds, it will fail at runtime, as some of the SQL used in miniflux is
postgresql-specific.

internal/database/sqlite.go

@@ -0,0 +1,26 @@
+//go:build sqlite
+
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package database // import "miniflux.app/v2/internal/database"
+
+import (
+	"database/sql"
+	"time"
+
+	_ "github.com/mattn/go-sqlite3"
+)
+
+// NewConnectionPool configures the database connection pool.
+func NewConnectionPool(dsn string, _, _ int, _ time.Duration) (*sql.DB, error) {
+	db, err := sql.Open("sqlite3", dsn)
+	if err != nil {
+		return nil, err
+	}
+	return db, nil
+}
+
+func getDriverStr() string {
+	return "sqlite3"
+}