haxe: fix head spec (#12156)

Formula/haxe.rb

@@ -4,7 +4,6 @@ class Haxe < Formula
   url "https://github.com/HaxeFoundation/haxe.git",
       :tag => "3.4.2",
       :revision => "890f8c70cf23ce6f9fe0fdd0ee514a9699433ca7"
-  head "https://github.com/HaxeFoundation/haxe.git", :branch => "development"
 
   bottle do
     cellar :any
@@ -13,6 +12,11 @@ class Haxe < Formula
     sha256 "07da6182e4e2482c941a0b7e5d3ec0bda852ab6702c0067153c02e0c3a3a1925" => :yosemite
   end
 
+  head do
+    url "https://github.com/HaxeFoundation/haxe.git", :branch => "development"
+    depends_on "opam" => :build
+  end
+
   depends_on "ocaml" => :build
   depends_on "camlp4" => :build
   depends_on "cmake" => :build
@@ -22,9 +26,16 @@ class Haxe < Formula
   def install
     # Build requires targets to be built in specific order
     ENV.deparallelize
-    args = ["OCAMLOPT=ocamlopt.opt"]
-    args << "ADD_REVISION=1" if build.head?
-    system "make", *args
+
+    if build.head?
+      ENV["OPAMROOT"] = buildpath/"opamroot"
+      ENV["OPAMYES"] = "1"
+      system "opam", "init", "--no-setup"
+      system "opam", "install", "ocamlfind"
+      system "opam", "config", "exec", "--", "make", "ADD_REVISION=1"
+    else
+      system "make", "OCAMLOPT=ocamlopt.opt"
+    end
 
     # Rebuild haxelib as a valid binary
     cd "extra/haxelib_src" do