add support for build tag to allow to disable some features

The following build tags are available:

- "nogcs", disable Google Cloud Storage backend
- "nos3", disable S3 Compabible Object Storage backends
- "nobolt", disable Bolt data provider
- "nomysql", disable MySQL data provider
- "nopgsql", disable PostgreSQL data provider
- "nosqlite", disable SQLite data provider
- "noportable", disable portable mode

dataprovider/sqlite.go

@@ -1,3 +1,5 @@
+// +build !nosqlite
+
 package dataprovider
 
 import (
@@ -6,6 +8,9 @@ import (
 	"path/filepath"
 	"strings"
 
+	// we import go-sqlite3 here to be able to disable SQLite support using a build tag
+	_ "github.com/mattn/go-sqlite3"
+
 	"github.com/drakkan/sftpgo/logger"
 	"github.com/drakkan/sftpgo/utils"
 )
@@ -41,6 +46,10 @@ type SQLiteProvider struct {
 	dbHandle *sql.DB
 }
 
+func init() {
+	utils.AddFeature("+sqlite")
+}
+
 func initializeSQLiteProvider(basePath string) error {
 	var err error
 	var connectionString string