depend on zstd 1.3.8

On debiln  stretch, you need to install that from debian backuports ...

debian/control

@@ -6,7 +6,7 @@ Build-Depends: bash-completion,
                debhelper (>= 10),
                python3-docutils,
 	       python3-sphinx,
-               libzstd-dev,
+               libzstd-dev (>= 1.3.8),
                libpam0g-dev,
                pkg-config,
 Standards-Version: 3.9.5
@@ -18,6 +18,6 @@ Depends: ${shlibs:Depends}, ${misc:Depends},
 	 libjs-extjs (>= 6.0.1),
 	 fonts-font-awesome,
 	 proxmox-widget-toolkit,
-	 libzstd1,
+	 libzstd1 (>= 1.3.8),
 Description: Proxmox Backup Server
  This is experimental code used to test Rust.