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_disabled.go

@@ -0,0 +1,17 @@
+// +build nosqlite
+
+package dataprovider
+
+import (
+	"errors"
+
+	"github.com/drakkan/sftpgo/utils"
+)
+
+func init() {
+	utils.AddFeature("-sqlite")
+}
+
+func initializeSQLiteProvider(basePath string) error {
+	return errors.New("SQLite disabled at build time")
+}