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/dump_journal.cpp

@@ -75,7 +75,7 @@ int main(int argc, char *argv[])
             uint64_t s;
             for (s = 0; s < self.journal_block; s += 8)
             {
-                if (*((uint64_t*)(data+s)) != 0)
+                if (*((uint64_t*)((uint8_t*)data+s)) != 0)
                     break;
             }
             if (s == self.journal_block)
@@ -139,7 +139,7 @@ int journal_dump_t::dump_block(void *buf)
     bool wrapped = false;
     while (pos < journal_block)
     {
-        journal_entry *je = (journal_entry*)(buf + pos);
+        journal_entry *je = (journal_entry*)((uint8_t*)buf + pos);
         if (je->magic != JOURNAL_MAGIC || je->type < JE_MIN || je->type > JE_MAX ||
             !all && started && je->crc32_prev != crc32_last)
         {