Build issue of consumers with gcc13

[heirecka]

Like some new versions before it, GCC 13 moves around some internal includes and <stdint.h>/<cstdint> seem no longer transitively included.
This becomes a problem when building trojita for example:

/usr/x86_64-pc-linux-gnu/include/mimetic/rfc822/messageid.h:29:23: error: expected ')' before 'thread_id'
   29 |     MessageId(uint32_t thread_id = 0 );
      |              ~        ^~~~~~~~~~
      |                       )

Even though messageid.h includes <stdint.h>, it does so behind a #ifdef HAVE_STDINT_H, which isn't defined anywhere in the installed headers.

[lukem]

I haven't noticed this using g++ 13.1.1 (on RHEL 8) with missing / <stdint.h>

I did experience various build warnings (especially with -Wall), which I fixed in various commits in pull request #30 .