Add infrastructure for using multiple event loops

This change adds two configuration options:

  --with-eventloop=[glib|internal|auto]
  --with-pollmethod=[epoll|auto]

First allows switching between GLib event loop and internal one.
Second chooses backend to use for internal event loop.
Conditional compilation symbols are changed accordingly.
Additional helper macro MPD_OPTIONAL_FUNC_NODEF is added as well.

src/event/MultiSocketMonitor.cxx

@@ -25,7 +25,7 @@
 
 #include <assert.h>
 
-#ifdef USE_EPOLL
+#ifdef USE_INTERNAL_EVENTLOOP
 
 MultiSocketMonitor::MultiSocketMonitor(EventLoop &_loop)
 	:IdleMonitor(_loop), TimeoutMonitor(_loop), ready(false) {
@@ -65,7 +65,9 @@ MultiSocketMonitor::OnIdle()
 	}
 }
 
-#else
+#endif
+
+#ifdef USE_GLIB_EVENTLOOP
 
 /**
  * The vtable for our GSource implementation.  Unfortunately, we