build: fix unused-parameter warnings

A couple changes to enable the use of -Wunused-parameter.

libmapper is built with the c compiler, which does not support omitting
the parameter name in the function definition, so use the de-facto
workaround and define an UNUSED() cpp macro.

Change-Id: I1c0d79d33097417fc3689351c4787e591fc23e9c
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/src/main.cpp b/src/main.cpp
index 4937057..be24368 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -468,8 +468,8 @@
 
     // Construct a signal set registered for process termination.
     boost::asio::signal_set signals(io, SIGINT, SIGTERM);
-    signals.async_wait([&io](const boost::system::error_code& error,
-                             int signal_number) { io.stop(); });
+    signals.async_wait(
+        [&io](const boost::system::error_code&, int) { io.stop(); });
 
     interface_map_type interface_map;
     boost::container::flat_map<std::string, std::string> name_owners;