conf-gmp v4 doesn't install anymore

[UnixJunkie]

opam v2.1.6

On an Ubuntu system, I get:

$ opam install conf-gmp
The following actions will be performed:
  - install conf-gmp 4

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
-> retrieved conf-gmp.4  (cached)
[ERROR] The compilation of conf-gmp.4 failed at "sh -exc cc -c $CFLAGS -I/usr/local/include test.c".

#=== ERROR while compiling conf-gmp.4 =========================================#
# context              2.1.6 | linux/x86_64 | ocaml-base-compiler.5.1.0 | git+https://github.com/ocaml/opam-repository.git
# path                 ~/.opam/5.1.0/.opam-switch/build/conf-gmp.4
# command              ~/.opam/opam-init/hooks/sandbox.sh build sh -exc cc -c $CFLAGS -I/usr/local/include test.c
# exit-code            1
# env-file             ~/.opam/log/conf-gmp-44933-0ee07a.env
# output-file          ~/.opam/log/conf-gmp-44933-0ee07a.out
### output ###
# bwrap: loopback: Failed RTM_NEWADDR: Operation not permitted

Workaround, since I know the system deps are included on this computer, I ran:

opam install --fake conf-gmp

[UnixJunkie]

almost same error for zarith v1.13

The following actions will be performed:
  - install zarith 1.13

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
-> retrieved zarith.1.13  (cached)
[ERROR] The compilation of zarith.1.13 failed at "./configure".

#=== ERROR while compiling zarith.1.13 ========================================#
# context     2.1.6 | linux/x86_64 | ocaml-base-compiler.5.1.0 | git+https://github.com/ocaml/opam-repository.git
# path        ~/.opam/5.1.0/.opam-switch/build/zarith.1.13
# command     ~/.opam/opam-init/hooks/sandbox.sh build ./configure
# exit-code   1
# env-file    ~/.opam/log/zarith-45474-593a2c.env
# output-file ~/.opam/log/zarith-45474-593a2c.out
### output ###
# bwrap: loopback: Failed RTM_NEWADDR: Operation not permitted

<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
+- The following actions failed
| - build zarith 1.13
+- 
- No changes have been performed