eval: re-add has('mac') and has('macunix') #976

They were removed but they can be handy to separate some things to do at
startup in a .vimrc for example.

src/nvim/eval.c

@@ -9703,6 +9703,10 @@ static void f_has(typval_T *argvars, typval_T *rettv)
     "lispindent",
     "listcmds",
     "localmap",
+#ifdef __APPLE__
+    "mac",
+    "macunix",
+#endif
     "menu",
     "mksession",
     "modify_fname",