Add support for installing system packages

[zoickx]

Some opam packages have system non-OCaml dependencies, managed by the system's package manager. As of now, coq-config will fail to install such dependencies, despite them being supported by default with opam depext since 2.1.0. See more in opam FAQ, GitHub repo.

[zoickx]

An example of a failed invocation looks like this (no user input)

$ coq-config

<><> Installing new switch packages <><><><><><><><><><><><><><><><><><><><><><>
Switch invariant: ["ocaml-base-compiler" {= "4.12.0"} | "ocaml-system" {= "4.12.0"}]

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
-> installed base-bigarray.base
-> installed base-threads.base
-> installed base-unix.base
-> installed ocaml-options-vanilla.1
-> retrieved ocaml-base-compiler.4.12.0  (https://opam.ocaml.org/cache)
-> installed ocaml-base-compiler.4.12.0
-> installed ocaml-config.2
-> installed ocaml.4.12.0
Done.
# Run eval $(opam env --switch=asn1-vst28) to update the current shell environment
[coq-released] Initialised
coq-vst-32 is now pinned to version 2.8
Package coq-struct-tact does not exist, create as a NEW package? [Y/n] y
[coq-struct-tact.~dev] synchronised (git+https://github.com/uwplse/StructTact.git)
coq-struct-tact is now pinned to git+https://github.com/uwplse/StructTact.git (version dev)

<><> Synchronising pinned packages ><><><><><><><><><><><><><><><><><><><><><><>
[coq-struct-tact.dev] synchronised (no changes)

The following actions will be performed:
  - install ocamlfind       1.9.1    [required by coq]
  - install conf-g++        1.0      [required by coq-flocq]
  - install conf-gmp        3        [required by zarith]
  - install conf-findutils  1        [required by coq]
  - install dune            2.9.0    [required by coq-struct-tact]
  - install num             1.4      [required by coq]
  - install zarith          1.12     [required by coq]
  - install menhirSdk       20210419 [required by menhir]
  - install menhirLib       20210419 [required by menhir]
  - install coq             8.13.2   [required by coq-struct-tact, coq-ext-lib, coq-vst-32]
  - install menhir          20210419 [required by coq-compcert-32]
  - install coq-struct-tact dev*
  - install coq-menhirlib   20210419 [required by coq-compcert-32]
  - install coq-flocq       3.4.2    [required by coq-vst-32]
  - install coq-ext-lib     0.11.3
  - install coq-compcert-32 3.9      [required by coq-vst-32]
  - install coq-vst-32      2.8*
===== 17 to install =====

The following system packages will first need to be installed:
    libgmp-dev

<><> Handling external dependencies <><><><><><><><><><><><><><><><><><><><><><>
Let opam run your package manager to install the required system packages?
(answer 'n' for other options) [Y/n] n
This command should get the requirements installed:

    apt-get install libgmp-dev

[NOTE] Use 'opam option depext-run-installs=false' if you don't want to be prompted again.
You can retry with '--assume-depexts' to skip this check, or run 'opam option depext=false' to permanently disable handling of system packages altogether.
Error installing pakages