I'm trying to compile bitcoind on Windows but build_unix for db-4.8.30.NC is empty. What do I need to do?
The build_unix directory contains just the .IGNORE_ME file in the distribution.  The directory is populated by the make command.

../dist/configure --disable-replication --enable-mingw --enable-cxx
make


You do not need to edit db.h in the latest distribution since they have already corrected the typedef.