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/proxy/prefetch.h

@@ -0,0 +1,14 @@
+#ifndef PREFETCH_H
+#define PREFETCH_H
+
+#define PREFETCH_BUFSIZE 4096
+
+struct prefetch {
+        int is_full;
+        __be64 from;
+        __be32 len;
+
+        char buffer[PREFETCH_BUFSIZE];
+};
+
+#endif