Support configuring compile flags using Elixir config

Author: dvic

README.md

@@ -74,6 +74,17 @@ You can enable certain features by doing the following:
 export EXQLITE_SYSTEM_CFLAGS=-DSQLITE_ENABLE_DBSTAT_VTAB=1
 ```
 
+Or you can pass extra environment variables using the Elixir config:
+
+```elixir
+config :exqlite,
+  force_build: true,
+  make_env: %{
+    "EXQLITE_SYSTEM_CFLAGS" => "-DSQLITE_ENABLE_DBSTAT_VTAB=1",
+    "V" => "1"
+  }
+```
+
 ### Listing Flags Used For Compilation
 
 If you `export V=1` the flags used for compilation will be output to stdout.

mix.exs

@@ -21,6 +21,7 @@ defmodule Exqlite.MixProject do
         versions: ["2.15", "2.16", "2.17"],
         availability: &target_available_for_nif_version?/2
       ],
+      make_env: Application.get_env(:exqlite, :make_env, %{}),
       cc_precompiler: cc_precompiler(),
       start_permanent: Mix.env() == :prod,
       aliases: aliases(),