Fix build under clang and some warnings

Build problems fixed:
- void* pointer arithmetic which is a GNU extension (works as byte*)
- "variable size object may not be initialized" which is OK under GCC
- nullptr_t related error in json11 (it lacks 'operator <' in clang)

Warnings fixed:
- empty nested struct initializer { 0 } replaced by {}
- removed several unused lambda captures

src/blockstore_impl.h

@@ -280,7 +280,7 @@ class blockstore_impl_t
 
     friend class blockstore_init_meta;
     friend class blockstore_init_journal;
-    friend class blockstore_journal_check_t;
+    friend struct blockstore_journal_check_t;
     friend class journal_flusher_t;
     friend class journal_flusher_co;