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imagelib is not compatible with ocaml 5

it fails with multiple errors about not linking unix.

```
=== ERROR while compiling imagelib.20200929 ==================================#
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/imagelib.20200929
command ~/.opam/opam-init/hooks/sandbox.sh build dune build -p imagelib -j 71 @install
exit-code 1
env-file ~/.opam/log/imagelib-7-975662.env
output-file ~/.opam/log/imagelib-7-975662.out
output ###
[...]
(cd _build/default && /home/opam/.opam/5.0/bin/ocamlopt.opt -w -40 -g -o app/imagetool.exe /home/opam/.opam/5.0/lib/stdlib-shims/stdlib_shims.cmxa /home/opam/.opam/5.0/lib/optint/optint.cmxa /home/opam/.opam/5.0/lib/checkseum/ocaml/checkseum_ocaml.cmxa /home/opam/.opam/5.0/lib/bigarray-compat/bigarray_compat.cmxa /home/opam/.opam/5.0/lib/decompress/de/de.cmxa /home/opam/.opam/5.0/lib/decompress/zl/zl.cmxa src/imagelib.cmxa unix/imagelib_unix.cmxa app/.imagetool.eobjs/native/dune__exe__Imagetool.cmx)
File "_none_", line 1:
Error: No implementations provided for the following modules:
Unix referenced from app/.imagetool.eobjs/native/dune__exe__Imagetool.cmx
```

Signed-off-by: Marcello Seri <marcello.seri@gmail.com>

Changed files
+6 -6
packages
imagelib
imagelib.20171028
imagelib.20180522
imagelib.20200929
imagelib.20210116
imagelib.20210402
imagelib.20210511
+1 -1
packages/imagelib/imagelib.20171028/opam
···
install: [make "install"]
remove: [make "uninstall"]
depends: [
-
"ocaml" {>= "4.03.0"}
"ocamlfind" {build}
"ocamlbuild" {build}
"decompress" {= "0.7"}
···
install: [make "install"]
remove: [make "uninstall"]
depends: [
+
"ocaml" {>= "4.03.0" & < "5.0"}
"ocamlfind" {build}
"ocamlbuild" {build}
"decompress" {= "0.7"}
+1 -1
packages/imagelib/imagelib.20180522/opam
···
install: [make "install"]
remove: [make "uninstall"]
depends: [
-
"ocaml" {>= "4.03.0"}
"ocamlfind" {build}
"ocamlbuild" {build}
"decompress" {= "0.7"}
···
install: [make "install"]
remove: [make "uninstall"]
depends: [
+
"ocaml" {>= "4.03.0" & < "5.0"}
"ocamlfind" {build}
"ocamlbuild" {build}
"decompress" {= "0.7"}
+1 -1
packages/imagelib/imagelib.20200929/opam
···
doc: "https://rlepigre.github.io/ocaml-imagelib"
depends: [
-
"ocaml" { >= "4.07.0" }
"base-unix"
"dune" { >= "2.3.0" }
"decompress" { >= "1.2.0" & < "1.3.0" }
···
doc: "https://rlepigre.github.io/ocaml-imagelib"
depends: [
+
"ocaml" { >= "4.07.0" & < "5.0" }
"base-unix"
"dune" { >= "2.3.0" }
"decompress" { >= "1.2.0" & < "1.3.0" }
+1 -1
packages/imagelib/imagelib.20210116/opam
···
doc: "https://rlepigre.github.io/ocaml-imagelib"
depends: [
-
"ocaml" { >= "4.07.0" }
"base-unix"
"dune" { >= "2.3.0" }
"decompress" { >= "1.2.0" & < "1.3.0" }
···
doc: "https://rlepigre.github.io/ocaml-imagelib"
depends: [
+
"ocaml" { >= "4.07.0" & < "5.0" }
"base-unix"
"dune" { >= "2.3.0" }
"decompress" { >= "1.2.0" & < "1.3.0" }
+1 -1
packages/imagelib/imagelib.20210402/opam
···
doc: "https://rlepigre.github.io/ocaml-imagelib"
depends: [
-
"ocaml" { >= "4.07.0" }
"base-unix"
"dune" { >= "2.3.0" }
"decompress" { >= "1.3.0" }
···
doc: "https://rlepigre.github.io/ocaml-imagelib"
depends: [
+
"ocaml" { >= "4.07.0" & < "5.0" }
"base-unix"
"dune" { >= "2.3.0" }
"decompress" { >= "1.3.0" }
+1 -1
packages/imagelib/imagelib.20210511/opam
···
doc: "https://rlepigre.github.io/ocaml-imagelib"
depends: [
-
"ocaml" { >= "4.07.0" }
"base-unix"
"dune" { >= "2.3.0" }
"decompress" { >= "1.3.0" }
···
doc: "https://rlepigre.github.io/ocaml-imagelib"
depends: [
+
"ocaml" { >= "4.07.0" & < "5.0" }
"base-unix"
"dune" { >= "2.3.0" }
"decompress" { >= "1.3.0" }