It looks like Trivial DB is shipped only with Samba itself nowadays and there is no separate project for it.
This means to compile TDB you need to download Samba sources and do the following:

1) run Samba's main ./configure;
2) run autogen.sh and ./configure in sources/tdb/ dir;

For some reason I also had to tweak Makefile manually in order to move "-I../include" to the beginning of CPPFLAGS,
otherwise wrong config.h was used (we need the global one).
After that `make` succeeded, but libtdb.a was NOT installed by `make install`, so I had to put it in place manually.

That's it, no more magic, --with-tdb=<TDB install prefix> should work after that.