getopt is not compatible with ocaml-5
```
=== ERROR while compiling getopt.20120615 ====================================#
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/getopt.20120615
command ~/.opam/opam-init/hooks/sandbox.sh build ocaml setup.ml -configure --prefix /home/opam/.opam/5.0
exit-code 2
env-file ~/.opam/log/getopt-7-1c7c2a.env
output-file ~/.opam/log/getopt-7-1c7c2a.out
File "/home/pveber/usr/ocamlbrew/ocaml-3.12.1/build/odb/install-oasis/oasis-0.3.0~rc3/src/oasis/OASISUtils.ml", line 46, characters 20-36:
Error: Unbound value String.lowercase
```
Signed-off-by: Marcello Seri <marcello.seri@gmail.com>