gnu: Add ocaml-either.

* gnu/packages/ocaml.scm (ocaml-either): New variable.

gnu/packages/ocaml.scm

@@ -7476,6 +7476,29 @@ (define-public ocaml-dune-build-info
 reporting the version from the version control system during development to
 get an precise reference of when the executable was built.")))
 
+(define-public ocaml-either
+  (package
+    (name "ocaml-either")
+    (version "1.0.0")
+    (source
+      (origin
+        (method git-fetch)
+        (uri (git-reference
+               (url "https://github.com/mirage/either")
+               (commit version)))
+        (file-name (git-file-name name version))
+        (sha256
+          (base32 "099p1m24vz5i0043zcfp88krzjsa2qbrphrm4bnx84gif5vgkxwm"))))
+    (build-system dune-build-system)
+    (arguments
+     ;; no tests
+     `(#:tests? #f))
+    (home-page "https://github.com/mirage/either")
+    (synopsis "Compatibility Either module")
+    (description "This library is a compatibility module for the Either module
+defined in OCaml 4.12.0.")
+    (license license:expat)))
+
 (define-public ocamlformat
   (package
     (name "ocamlformat")