clips: fix darwin build

pkgs/development/interpreters/clips/default.nix

@@ -3,18 +3,26 @@
 stdenv.mkDerivation rec {
   version = "6.31";
   pname = "clips";
+
   src = fetchurl {
     url = "mirror://sourceforge/clipsrules/CLIPS/${version}/clips_core_source_${
         builtins.replaceStrings [ "." ] [ "" ] version
       }.tar.gz";
     sha256 = "165k0z7dsv04q432sanmw0jxmxwf56cnhsdfw5ffjqxd3lzkjnv6";
   };
+
+  postPatch = ''
+    substituteInPlace core/makefile --replace 'gcc' '${stdenv.cc.targetPrefix}cc'
+  '';
+
   makeFlags = [ "-C" "core" ];
+
   installPhase = ''
     runHook preInstall
     install -D -t $out/bin core/clips
     runHook postInstall
   '';
+
   meta = with lib; {
     description = "A Tool for Building Expert Systems";
     homepage = "http://www.clipsrules.net/";
@@ -26,6 +34,6 @@ stdenv.mkDerivation rec {
     '';
     license = licenses.publicDomain;
     maintainers = [ maintainers.league ];
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }