ocamlPackages.ocaml_extlib: add support for OCaml 4.12
ocamlPackages.ocaml_extlib: add support for OCaml 4.12

Pull in patches from https://github.com/ygrek/ocaml-extlib/pull/55
which add support for OCaml 4.12. These patches are also included in
extlib 1.7.8, however extlib 1.7.8 includes some API changes which break
for example google-drive-ocamlfuse.

Therefore we follow the opam repository and patch our default extlib
version: https://github.com/ocaml/opam-repository/pull/18041

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs