Deprecate, rather than make unavailable

packages/conf-zstd/conf-zstd.0.1/opam

@@ -4,11 +4,18 @@ homepage: "http://zstd.net/"
 bug-reports: "https://github.com/ocaml/opam-repository/issues"
 authors: ["Facebook"]
 license: "BSD-3-Clause"
+build: [["pkg-config" "libzstd"]]
+depends: ["conf-pkg-config" {build}]
+depexts: [
+  ["libzstd-dev"] {os-family = "debian"}
+  ["libzstd-devel"] {os-distribution = "centos"}
+  ["libzstd-devel"] {os-distribution = "rhel"}
+  ["libzstd-devel"] {os-distribution = "fedora"}
+  ["zstd-dev"] {os-distribution = "alpine"}
+  ["zstd"] {os-distribution = "homebrew" & os = "macos"}
+  ["zstd"] {os = "win32" & os-distribution = "cygwinports"}
+]
 synopsis: "Virtual package relying on zstd (legacy)"
 description: "This package version has been deprecated - use conf-zstd.1.3.8."
 flags: [ conf deprecated ]
-# This package has been disabled in order to simplify the maintenance of
-# conf-zstd. conf-zstd.1.3.8 was added to "encode" a build requirement of at
-# least version 1.3.8, but this should have been done using an entirely new
-# package.
-available: false
+available: os != "win32" | os-distribution = "cygwinports"