Well... there are a lot of distribution based on rpm. Which one are you running on? First, you should try to install the rpmbuild package provided by the official repository of your distro (e.g. by means of Add/Remove software or Yast, or yum, or zypper....) so that it will take care of dependencies automatically.
Edit: I checked on my system (Opensuse 11.1) and the librpmbuild shared library is provided by the rpm package (as well as the rpmbuild command). I suspect you have downloaded a wrong version of rpmbuild and the shared library is not where it is expected to be. You can verify the required libraries using
Code:
ldd $(which rpmbuild)
in any case I will stick with the official packages.