ploticus: add Darwin support

thanks @risicle

pkgs/tools/graphics/ploticus/default.nix

@@ -45,6 +45,7 @@ stdenv.mkDerivation rec {
   preBuild = ''
     cd src
   '';
+  makeFlags = [ "CC=cc" ];
 
   preInstall = ''
     mkdir -p "$out/bin"
@@ -74,6 +75,6 @@ stdenv.mkDerivation rec {
     license = licenses.gpl2Plus;
     maintainers = with maintainers; [ pSub ];
     homepage = "http://ploticus.sourceforge.net/";
-    platforms = with platforms; linux;
+    platforms = with platforms; linux ++ darwin;
   };
 }