add gmp as overlay

packages/gmp.6.2.1-3/opam

@@ -0,0 +1,23 @@
+opam-version: "2.0"
+maintainer: "Lucas Pluvinage <lucas@tarides.com>"
+license: ["LGPL-3.0-only" "LGPL-2.0-only"]
+authors: "Torbjörn Granlund and contributors"
+homepage: "https://github.com/mirage/ocaml-gmp"
+bug-reports: "https://github.com/mirage/ocaml-gmp/issues"
+dev-repo: "git+https://github.com/mirage/ocaml-gmp.git"
+substs: [ "src/build.sh" ]
+build: [
+ [ "dune" "build" "-p" name "-j" jobs ]
+ [ "dune" "runtest" "-p" name "-j" jobs ] {with-test}
+]
+depends: [
+  "ocaml" {>= "4.02.0"}
+  "dune" {>= "2.6"}
+  "conf-m4"
+]
+synopsis: "The GNU Multiple Precision Arithmetic Library"
+description: """Dune packaging of the GMP library, suitable for 
+cross-compilation."""
+url {
+  src: "https://github.com/hannesm/ocaml-gmp.git#freebsd"
+}