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

vfs/gcsfs.go

@@ -1,3 +1,5 @@
+// +build !nogcs
+
 package vfs
 
 import (
@@ -19,6 +21,7 @@ import (
 
 	"github.com/drakkan/sftpgo/logger"
 	"github.com/drakkan/sftpgo/metrics"
+	"github.com/drakkan/sftpgo/utils"
 )
 
 var (
@@ -27,22 +30,6 @@ var (
 	gcsDefaultFieldsSelection = []string{"Name", "Size", "Deleted", "Updated"}
 )
 
-// GCSFsConfig defines the configuration for Google Cloud Storage based filesystem
-type GCSFsConfig struct {
-	Bucket string `json:"bucket,omitempty"`
-	// KeyPrefix is similar to a chroot directory for local filesystem.
-	// If specified the SFTP user will only see objects that starts with
-	// this prefix and so you can restrict access to a specific virtual
-	// folder. The prefix, if not empty, must not start with "/" and must
-	// end with "/".
-	// If empty the whole bucket contents will be available
-	KeyPrefix            string `json:"key_prefix,omitempty"`
-	CredentialFile       string `json:"-"`
-	Credentials          string `json:"credentials,omitempty"`
-	AutomaticCredentials int    `json:"automatic_credentials,omitempty"`
-	StorageClass         string `json:"storage_class,omitempty"`
-}
-
 // GCSFs is a Fs implementation for Google Cloud Storage.
 type GCSFs struct {
 	connectionID   string
@@ -53,6 +40,10 @@ type GCSFs struct {
 	ctxLongTimeout time.Duration
 }
 
+func init() {
+	utils.AddFeature("+gcs")
+}
+
 // NewGCSFs returns an GCSFs object that allows to interact with Google Cloud Storage
 func NewGCSFs(connectionID, localTempDir string, config GCSFsConfig) (Fs, error) {
 	var err error