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/message/mu-document.hh

@@ -19,7 +19,6 @@
 #ifndef MU_DOCUMENT_HH__
 #define MU_DOCUMENT_HH__
 
-#include <xapian.h>
 #include <utility>
 #include <string>
 #include <vector>