gnu: ocaml4.02-ppx-deriving: Don't use unstable tarball.

* gnu/packages/ocaml.scm (ocaml4.02-ppx-deriving)[source]: Use GIT-FETCH and
GIT-FILE-NAME.
[arguments]: Add ‘make-git-checkout-writable’ phase.
Fix indentation.

gnu/packages/ocaml.scm

@@ -2949,12 +2949,13 @@ (define-public ocaml4.02-ppx-deriving
     (version "4.1")
     (source
      (origin
-        (method url-fetch)
-        (uri (string-append "https://github.com/whitequark/ppx_deriving//archive/v"
-                            version ".tar.gz"))
-        (sha256 (base32
-                  "1fr16g121j6zinwcprzlhx2py4271n9jzs2m9hq2f3qli2b1p0vl"))
-        (file-name (string-append name "-" version ".tar.gz"))))
+       (method git-fetch)
+       (uri (git-reference
+             (url "https://github.com/whitequark/ppx_deriving.git")
+             (commit (string-append "v" version))))
+       (file-name (git-file-name name version))
+       (sha256
+        (base32 "0cy9p8d8cbcxvqyyv8fz2z9ypi121zrgaamdlp4ld9f3jnwz7my9"))))
     (build-system ocaml-build-system)
     (native-inputs
      `(("js-build-tools" ,ocaml4.02-js-build-tools)
@@ -2969,6 +2970,10 @@ (define-public ocaml4.02-ppx-deriving
        #:findlib ,ocaml4.02-findlib
        #:phases
        (modify-phases %standard-phases
+         (add-after 'unpack 'make-git-checkout-writable
+           (lambda _
+             (for-each make-file-writable (find-files "."))
+             #t))
          (delete 'configure)
          (add-before 'install 'fix-environment
            (lambda* (#:key outputs #:allow-other-keys)