Extend conf-gmp-paths to support OpenSuse

[jmid]

Spotted as apronext failures on opensuse-tumbleweed as part of #28224:

#=== ERROR while compiling conf-gmp-paths.1 ===================================#
# context              2.3.0 | linux/x86_64 | ocaml-base-compiler.4.14.2 | file:///home/opam/opam-repository
# path                 ~/.opam/4.14/.opam-switch/build/conf-gmp-paths.1
# command              ~/.opam/opam-init/hooks/sandbox.sh build sh /home/opam/.opam/4.14/lib/ez-conf-lib/ez-conf-lib gmp gmp.h test-gmp.c --package-name conf-gmp-paths -- /usr/local
# exit-code            1
# env-file             ~/.opam/log/conf-gmp-paths-7-fc9a11.env
# output-file          ~/.opam/log/conf-gmp-paths-7-fc9a11.out
### output ###
# checking compilation with gcc -O2 -fno-strict-aliasing -fwrapv -pthread -fPIC  -D_FILE_OFFSET_BITS=64 : working
# looking for gmp without prefix
# include gmp.h: not found
# looking for gmp in prefix /usr/local
# looking for gmp in prefix /usr
# looking for gmp in prefix /usr/local
# library gmp not found
# gmp not found; pass further possible prefixes     	 after '--' on the command line

This makes the line identical to the corresponding suse-line in packages/conf-gmp/conf-gmp.5/opam.

[jmid]

All green except for Windows and a lint error. Should be good to go.
Then we can rebase and rerun the CI for #28224.