gsl is not compatible with ocaml-5
Old versions fails with
```
=== ERROR while compiling gsl.0.18.5 =========================================#
context 2.2.0~alpha~dev | linux/x86_64 | ocaml-base-compiler.5.0.0 | file:///home/opam/opam-repository
path ~/.opam/5.0/.opam-switch/build/gsl.1.18.5
command ~/.opam/opam-init/hooks/sandbox.sh build ocaml setup.ml -configure --prefix /home/opam/.opam/5.0 --disable-camlp4
exit-code 2
env-file ~/.opam/log/gsl-7-d5bd64.env
output-file ~/.opam/log/gsl-7-d5bd64.out
File "./setup.ml", line 318, characters 20-36:
318 | String.compare (String.lowercase s1) (String.lowercase s2)
^^^^^^^^^^^^^^^^
Error: Unbound value String.lowercase
```
while newer ones fail with c-bindings errors like
```
wrappers.h:36:7: warning: implicit declaration of function 'alloc_small'; did you mean 'caml_alloc_small'? [-Wimplicit-function-declaration]
36 | r = alloc_small(2, 0);
| ^~~~~~~~~~~
| caml_alloc_small
```
Signed-off-by: Marcello Seri <marcello.seri@gmail.com>