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/mysql.go

@@ -1,3 +1,5 @@
+// +build !nomysql
+
 package dataprovider
 
 import (
@@ -6,7 +8,11 @@ import (
 	"strings"
 	"time"
 
+	// we import go-sql-driver/mysql here to be able to disable MySQL support using a build tag
+	_ "github.com/go-sql-driver/mysql"
+
 	"github.com/drakkan/sftpgo/logger"
+	"github.com/drakkan/sftpgo/utils"
 )
 
 const (
@@ -28,6 +34,10 @@ type MySQLProvider struct {
 	dbHandle *sql.DB
 }
 
+func init() {
+	utils.AddFeature("+mysql")
+}
+
 func initializeMySQLProvider() error {
 	var err error
 	logSender = fmt.Sprintf("dataprovider_%v", MySQLDataProviderName)