czmq: add test (#993)

Adaptation of the Hello World! example from http://czmq.zeromq.org/ that
uses an in-process channel, sends a string, receives it, and prints it.

Formula/czmq.rb

@@ -46,4 +46,34 @@ class Czmq < Formula
     system "make", "install"
     rm Dir["#{bin}/*.gsl"]
   end
+
+  test do
+    (testpath/"test.c").write <<-EOS.undent
+      #include <czmq.h>
+
+      int main(void)
+      {
+        zsock_t *push = zsock_new_push("inproc://hello-world");
+        zsock_t *pull = zsock_new_pull("inproc://hello-world");
+
+        zstr_send(push, "Hello, World!");
+        char *string = zstr_recv(pull);
+        puts(string);
+        zstr_free(&string);
+
+        zsock_destroy(&pull);
+        zsock_destroy(&push);
+
+        return 0;
+      }
+    EOS
+
+    flags = ENV.cflags.to_s.split + %W[
+      -I#{include}
+      -L#{lib}
+      -lczmq
+    ]
+    system ENV.cc, "-o", "test", "test.c", *flags
+    assert_equal "Hello, World!\n", shell_output("./test")
+  end
 end