There are systems out there, wich don't have ndbm, but gdbm and ndbm.h directly in the include-path (usually /usr/include), not in the subdirectory i.e. gdbm.

setup.py at the moment assumes, that there is ndbm on the system, if there is ndbm.h in the include path. If it doesn't find libndbm, it continues to try to use ndbm without library, which doesn't work.
BLFS (http://www.linuxfromscratch.org/blfs/view/svn/general/python.html) fills in the gdbm and gdbm-compat library, which works, but is wrong, because we really use gdbm and not ndbm.
The right way is to start with the library change and then look for the header file, as it is in the proposed patch for setup.py