Switch from a rake-based build to a make-based build.

This commit beefs up the Makefile to do the build, instead of the
Rakefile.

It also removes from the Rakefile the dependency on rake_utils, which
should mean it's ok to build in a schroot.

The files are reorganised to make the Makefile rules more tractable,
although the reorganisation reveals a problem with our current code
organisation.

The problem is that the proxy-specific code transitively depends on the
server code via flexnbd.h, which has a circular dependency on the server
and client structs. This should be broken in a future commit by
separating the flexnbd struct into a shared config struct and
server-specific parts, so that the server code can be moved into
src/server to more accurately show the functional dependencies.

src/common/mbox.h

@@ -0,0 +1,55 @@
+#ifndef MBOX_H
+#define MBOX_H
+
+/** mbox
+ * A thread sync object.  Put a void * into the mbox in one thread, and
+ * get it out in another.  The receiving thread will block if there's
+ * nothing in the mbox, and the sending thread will block if there is.
+ * The mbox doesn't assume any responsibility for the pointer it's
+ * passed - you must free it yourself if it's malloced.
+ */
+
+
+#include <pthread.h>
+
+
+struct mbox {
+	void * contents;
+
+	/** Marker to tell us if there's content in the box.
+	 * Keeping this separate allows us to use NULL for the contents.
+	 */
+	int full;
+
+	/** This gets signaled by mbox_post, and waited on by
+	 * mbox_receive */
+	pthread_cond_t filled_cond;
+	/** This is signaled by mbox_receive, and waited on by mbox_post */
+	pthread_cond_t emptied_cond;
+	pthread_mutex_t mutex;
+};
+
+
+/* Create an mbox. */
+struct mbox * mbox_create(void);
+
+/* Put something in the mbox, blocking if it's already full.
+ * That something can be NULL if you want.
+ */
+void mbox_post( struct mbox *, void *);
+
+/* See what's in the mbox.  This isn't thread-safe. */
+void * mbox_contents( struct mbox *);
+
+/* See if anything has been put into the mbox.  This isn't thread-safe.
+ * */
+int mbox_is_full( struct mbox *);
+
+/* Get the contents from the mbox, blocking if there's nothing there. */
+void * mbox_receive( struct mbox *);
+
+/* Free the mbox and destroy the associated pthread bits. */
+void mbox_destroy( struct mbox *);
+
+
+#endif