Earlier in the chapter, the class Unix tools ar and ranlib were used to build static libraries, and gcc was used to build a dynamic library. Mac OS X has a tool, libtool, which is peculiar to the platform, but provides a superset of features over ar and friends. The GNU project also has a libtool, but it is unrelated to the one in Mac OS X.
Make a static library like this and use it:
libtool -static -o libaddum.a src?.o$
gcc -g -o useadd useadd.m -L. -laddum
And dynamic libraries (but not bundles) can be created thusly:
libtool -dynamic -macosx_version_min 10.5 -o libaddum.dylib ...