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/osd_primary_write.cpp

@@ -113,7 +113,7 @@ resume_3:
             op_data->stripes[0].write_end != bs_block_size))
         {
             memcpy(
-                op_data->stripes[0].read_buf + op_data->stripes[0].req_start,
+                (uint8_t*)op_data->stripes[0].read_buf + op_data->stripes[0].req_start,
                 op_data->stripes[0].write_buf,
                 op_data->stripes[0].req_end - op_data->stripes[0].req_start
             );