after having sucessfully worked with rpm-2.5.2 under solaris I tried to build rpm-3.0.3. For now, the problems are related to misc/err.c. The configure script find out correctly that solaris 2.6 hasn't an err() function in the system libraries and tries to build it. This fails because of a lacking features.h that is included by err.c. After installing a trivial features.h the make process runs until trying to link dump. This has an unresolved err symbol. This in turn is in misc/libmisc.a, which is not linked with. When linking dump manually with libmisc.a, it fails with unresolved symbols in err.o such as __progname, __set_errno and fputs_unlocked. Looks like a linux/glibc-ism.
Um, I think you are using a preliminary version of rpm-3.0.3. Try getting the final release from ftp.rpm.org -- I ripped out err.h et al before releasing rpm-3.0.3 for the reasons you describe above.
I got it off the RedHat Linux 6.1 SRPM CD. I'll try to download the mentioned version now.