lib: centralize xapian include, enable move semantics

Only include xapian.h in one place, so we can have consistent options.
With that in place, we can enable C++ move semantics.

We don't do anything with that yet, but we check in the meson.build file
to see if we have the required xapian version.

mu/mu-cmd-info.cc

@@ -21,7 +21,6 @@
 
 #include "mu-cmd.hh"
 #include <message/mu-message.hh>
-#include <xapian.h>
 #include "utils/mu-utils.hh"
 
 #include <glib.h>