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-xapian-db.hh

@@ -29,10 +29,17 @@
 
 #include <glib.h>
 
-#include <xapian.h>
 #include <utils/mu-result.hh>
 #include <utils/mu-utils.hh>
 
+/* starting with 1.4.6, Xapian supports C++ move semantics,
+ * but only with XAPIAN_MOVE_SEMANTICS defined
+ */
+#ifndef XAPIAN_MOVE_SEMANTICS
+#define XAPIAN_MOVE_SEMANTICS
+#endif /*XAPIAN_MOVE_SEMANTICS*/
+#include <xapian.h>
+
 namespace Mu {
 
 // LCOV_EXCL_START