Makefile: simpler/faster "make help"

Using obscure black magic introduced in eaa2ddbb :)

In an out-of-tree directory, running "../configure && make help" will generate
some required files (.mak), then clone some submodules, compile at least
the capstone submodule, generate QMP and Trace files, and finally display
the help.

On an outdated computer (Sun Blade workstation), running "make help" took
more than 5h :) With this patch it took roughly 37min.

Suggested-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20171108032052.20029-1-f4bug@amsat.org>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>