Ok, I got that. In which-2.13/Makefile there is a line
LIBS = /usr/lib/libiberty.a
Compiling "which" alone says indeed gcc -g -O2 -o which getopt.o getopt1.o bash.o which.o ./tilde/libtilde.a /usr/lib/libiberty.a /usr/lib64/gcc/x86_64-suse-linux/4.1.0/../../../../x86_64-suse-linux/bin/ld: warning: i386 architecture of input file `/usr/lib/libiberty.a(xmalloc.o)' is incompatible with i386:x86-64 output
When changing this into LIBS = /usr/lib64/libiberty.a
the compiled "which" works fine. What's the best way to have that fixed for the next release of util-linux (if any) for 10.1? Report it to bugzilla?