Problem: test case is using internal API

It is poor style for test cases to use the internal API (i.e.
libzmq classes or header files), as this code serves the purpose
of teaching developers how to use the library (it doesn't do this
very well, it's an ambition).

Also, including headers in src/ creates problems when compiling.

Solution: remove use of src/macros.hpp.

tests/test_msg_ffn.cpp

@@ -27,12 +27,11 @@
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#include "macros.hpp"
 #include "testutil.hpp"
 
 void ffn(void *data, void *hint) {
-    LIBZMQ_UNUSED (data);
     // Signal that ffn has been called by writing "freed" to hint
+    (void) data;      //  Suppress 'unused' warnings at compile time
     memcpy(hint, (void *) "freed", 5);
 }