If you try to rebuild the runtime and during the make you find out that you are missing libz.o here it is what you have to do to create it.
You need to add the following "target" to the Makefile you are using to rebuild the runtime. Preferably at the end of the file:
$(CC) $(SHAREDLIB_LDFLAGS) $(NO_UNDEFS_LDFLAGS) $(LIBRUNCBL_LDFLAGS) \
libz.a -o libz.$(SHARED_LIB_EXT)
The second and third line you are adding have to start with a TAB, no spaces. It is compulsory for any make, so please be expecially careful when adding those lines.
After this addition, save the file, close it and type :
If you have renamed your Makefile, for exemple Makefile.ora type the following line
make -f Makefile.or libz.so
Now you only need to rebuild the runtime (make) and everything should work fine.