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_init.h

@@ -6,7 +6,7 @@
 class blockstore_init_meta
 {
     blockstore_impl_t *bs;
-    int wait_state = 0, wait_count = 0;
+    int wait_state = 0;
     bool zero_on_init = false;
     void *metadata_buffer = NULL;
     uint64_t metadata_read = 0;