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.

lib/mu-config.hh

@@ -29,8 +29,6 @@
 #include <variant>
 #include <unordered_map>
 
-#include <xapian.h>
-
 #include "mu-xapian-db.hh"
 
 #include <utils/mu-utils.hh>