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/s3fs_disabled.go

@@ -0,0 +1,18 @@
+// +build nos3
+
+package vfs
+
+import (
+	"errors"
+
+	"github.com/drakkan/sftpgo/utils"
+)
+
+func init() {
+	utils.AddFeature("-s3")
+}
+
+// NewS3Fs returns an error, S3 is disabled
+func NewS3Fs(connectionID, localTempDir string, config S3FsConfig) (Fs, error) {
+	return nil, errors.New("S3 disabled at build time")
+}