The C standard defines the difference between two (char*) pointers is
defined to be either short, int or long. Not long long (64 bits). On
64-bit systems, logging "%"PRIhttp_size_t (int64_t) for pointer
difference works by coincidence because a long or int is 64 bits, but
causes SEGV on 32 bit platforms.
The portable solution is to log pointer differences using "%d" and
explicitly cast the pointer difference to (int) in the arg list, or
"%ld" and cast it to (long).
So that "http_server.h" does not have to include "serval.h" which
creates a circular dependency.
Remove the __SERVALDNA__HTTP_SERVER_IMPLEMENTATION hack from
"http_server.h"
