opam: build with gmake on mac

The Makefile prohibits the default make found on MacOS, so
`opam install` won't work by default, unless the user specifies
to use gmake by eg setting opam's make env var.

It seems reasonable to use gmake by default on MacOS. I added similar
logic to karamel a few months ago.

Author: amosr-msft

pulse.opam

@@ -8,10 +8,15 @@ depends: [
   "ocaml"
   "fstar" {>= "2023.04.15~dev"}
 ]
+# require modern GNU make (brew package 'make') on mac
+depexts: ["coreutils" "make"] {os = "macos" & os-distribution = "homebrew"}
 build: [
+   # Makefile requires modern make - build with gmake on mac
+  ["gmake" "-j" jobs "ADMIT=1"] { os = "macos" & make = "make" }
   [make "-j" jobs "ADMIT=1"]
 ]
 install: [
+  ["gmake" "PREFIX=%{prefix}%" "install"] { os = "macos" & make = "make" }
   [make "PREFIX=%{prefix}%" "install"]
 ]
 dev-repo: "git://github.com/FStarLang/pulse"