show cxlc stderr on nonzero return code

tests/spec/main_spec.rb

@@ -4,6 +4,9 @@ require "open3"
 describe "cxlc" do
   def run_test(test_name)
     stdout, stderr, status = Open3.capture3("../build/cxlc", "#{test_name}.cxl")
+    if status != 0
+      $stderr.write(stderr)
+    end
     expect(status).to eq(0)
     stdout, stderr, status = Open3.capture3("gcc", "-o", test_name, "#{test_name}.c")
     if status != 0