dependency build fails on devuan beowulf + musl

[mro]

$ opam install http-lwt-client
The following actions will be performed:
  ∗ install zarith           1.12   [required by asn1-combinators, mirage-crypto-pk]
  ∗ install mirage-crypto-pk 0.10.3 [required by tls]
  ∗ install asn1-combinators 0.2.6  [required by x509]
  ∗ install x509             0.14.1 [required by ca-certs, tls]
  ∗ install tls              0.14.0 [required by http-lwt-client]
  ∗ install ca-certs         0.2.1  [required by http-lwt-client]
  ∗ install http-lwt-client  0.0.1
===== ∗ 7 =====
Do you want to continue? [Y/n] Y

<><> Gathering sources ><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
[asn1-combinators.0.2.6] found in cache
[ca-certs.0.2.1] found in cache
[http-lwt-client.0.0.1] found in cache
[mirage-crypto-pk.0.10.3] found in cache
[tls.0.14.0] found in cache
[x509.0.14.1] found in cache
[zarith.1.12] found in cache

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
[ERROR] The compilation of zarith failed at "/home/mro/.opam/opam-init/hooks/sandbox.sh build make".

#=== ERROR while compiling zarith.1.12 ========================================#
# context     2.0.3 | linux/x86_64 | ocaml-variants.4.10.0+musl+static+flambda | https://opam.ocaml.org#88233ad0
# path        ~/.opam/4.10.0+musl+static+flambda/.opam-switch/build/zarith.1.12
# command     ~/.opam/opam-init/hooks/sandbox.sh build make
# exit-code   2
# env-file    ~/.opam/log/zarith-11434-8b9749.env
# output-file ~/.opam/log/zarith-11434-8b9749.out
### output ###
# [...]
# ocamlc -I +compiler-libs -bin-annot  -c z.ml
# ocamlc -I +compiler-libs -bin-annot  -c q.mli
# ocamlc -I +compiler-libs -bin-annot  -c q.ml
# ocamlc -I +compiler-libs -bin-annot  -c big_int_Z.mli
# ocamlc -I +compiler-libs -bin-annot  -c big_int_Z.ml
# ocamlmklib -failsafe -o zarith zarith_version.cmo z.cmo q.cmo big_int_Z.cmo -lgmp
# ocamlc -ccopt "-I/home/mro/.opam/4.10.0+musl+static+flambda/lib/ocaml  -DHAS_GMP  -O3 -Wall -Wextra " -c caml_z.c
# caml_z.c:32:10: fatal error: gmp.h: No such file or directory
#  #include <gmp.h>
#           ^~~~~~~
# compilation terminated.
# make: *** [project.mak:141: caml_z.o] Error 2

<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
┌─ The following actions failed
│ λ build zarith 1.12
└─ 
╶─ No changes have been performed
# Run eval (opam env) to update the current shell environment

via https://discuss.ocaml.org/t/ann-http-client-library/8428

[hannesm]

That's best be reported to the zarith repository: https://github.com/ocaml/Zarith/

[mro]

I see, groundhog day: https://github.com/ocaml/Zarith/issues/77#issuecomment-706584811

[mro]

@nberth seems to have a recipe at https://github.com/nberth/mlgmpidl/issues/11#issuecomment-884881150