lib+guile: use Mu::Option, not std::optional

We need the extensions, and/or let's use _one_ optional implementation everywhere.

lib/mu-query.hh

@@ -21,12 +21,12 @@
 #define __MU_QUERY_HH__
 
 #include <memory>
-#include <optional>
 
 #include <glib.h>
 #include <mu-store.hh>
 #include <mu-query-results.hh>
 #include <utils/mu-utils.hh>
+#include <utils/mu-option.hh>
 #include <message/mu-message.hh>
 
 namespace Mu {
@@ -44,10 +44,10 @@ public:
 	 * @return the query-results, or Nothing in case of error.
 	 */
 
-	Option<QueryResults> run(const std::string&		expr	     = "",
-				 std::optional<Field::Id>	sortfield_id = {},
-				 QueryFlags			flags	     = QueryFlags::None,
-				 size_t				maxnum	     = 0) const;
+	Option<QueryResults> run(const std::string&	expr	     = "",
+				 Option<Field::Id>	sortfield_id = {},
+				 QueryFlags		flags	     = QueryFlags::None,
+				 size_t			maxnum	     = 0) const;
 
 	/**
 	 * run a Xapian query to count the number of matches; for the syntax, please