nif: add support for CFLAGS

mix.exs

@@ -51,6 +51,12 @@ defmodule Mix.Tasks.Compile.Nif do
     exts = Keyword.get(config, :exts, [:c])
     compiler = Keyword.get(config, :compilers, ["cc", "gcc", "clang"]) |> find_compiler
 
+    if cflags = System.get_env("CFLAGS") do
+      # FIXME: this isn't going to work too well with quoted spaces
+      # inside arguments
+      flags = [flags, String.split(cflags)]
+    end
+
     # Create the directory (e.g. "priv/")
     File.mkdir_p!(Path.dirname(file))