Compile c files in parallel (#14)

on my machine (24 cores) takes `mix compile` time down from ~70s to ~20s

Author: magnetised

README.md

@@ -30,7 +30,7 @@ dependencies in `mix.exs`:
 ```elixir
 def deps do
   [
-    {:pg_query_ex, "~> 0.7"}
+    {:pg_query_ex, "~> 0.8"}
   ]
 end
 ```

mix.exs

@@ -7,6 +7,7 @@ defmodule PgQuery.MixProject do
       elixir: "~> 1.13",
       version: "0.8.0",
       start_permanent: Mix.env() == :prod,
+      make_args: ["--jobs", "#{System.schedulers_online()}"],
       compilers: [:elixir_make] ++ Mix.compilers(),
       deps: deps(),
       description: description(),