And Mac OS X 10.5 running on Intel hardware is Unix (with eye candy on top). It’s passed the tests and bears the Unix mark, licensed from the Open Group. Furthermore, I also run Windows XP on the same machine. So my Mac is a PC and also a Unix workstation.
It’s usually due to templates. STL as well as the Boost libraries have a good deal of functionality in the header files, though they still have some code in libraries that are linked in at link- or load-time.