update to use fmt-based apis

Not complete, but a first big stab converting users of Mu::Error and
various g_warning & friends, format to the new libfmt-based APIs.

guile/mu-guile.hh

@@ -21,9 +21,14 @@
 #define __MU_GUILE_H__
 
 #include <glib.h>
-#include <libguile.h>
 #include <mu-query.hh>
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wredundant-decls"
+#pragma GCC diagnostic ignored "-Wvolatile"
+#include <libguile.h>
+#pragma GCC diagnostic pop
+
 /**
  * get the singleton Store instance
  */