To port KDE to a system other than UNIX, you need to provide implementations of any functions used by, but not provided by, KDE. This means looking to see which functions are used and providing #ifdefs etc. Some parts of KDE will, of course, need to be completely re-designed.
My humble opinion is that to make porting a success, a new source module should be created. This should implementations and stubs for each platform, plus a unified API which proxies these implementations. Perhaps someone with more experience of porting knows a better way ? Personally, I hate #ifdefs, so this idea is my gut reaction to the problem.
Here's a list of headers used by various source files in kdelibs. Note that this list isn't perfect - you may find some headers which actually belong to KDE. Please let me know so I can reduce this list !
Stuff like libxslt, openssl etc. might already be ported to other platforms, but I haven't checked yet.
X11/ICE/ICE.h X11/ICE/ICEconn.h X11/ICE/ICElib.h X11/ICE/ICEmsg.h X11/ICE/ICEproto.h X11/ICE/ICEutil.h X11/SM/SMlib.h X11/X.h X11/Xatom.h X11/Xfuncproto.h X11/Xfuncs.h X11/Xlib.h X11/Xlibint.h X11/Xmd.h X11/Xos.h X11/Xproto.h X11/Xutil.h X11/Xw32defs.h X11/Xwinsock.h X11/extensions/XShm.h X11/keysym.h X11/keysymdef.h alloca.h ansidecl.h argz.h arpa/inet.h arpa/nameser.h audio/audiolib.h awe_voice.h connect.h ctype.h cups/cups.h cups/http.h cups/ipp.h cups/language.h cups/ppd.h cupslist.h db.h debug.h dirent.h dl.h dld.h dlfcn.h errno.h fam.h fcntl.h features.h float.h fstab.h glib.h gmain.h grp.h ieeefp.h io.h iomanager.h jpeglib.h kernel/image.h lan/in.h libaudioio.h libio.h libutil.h libxslt/transform.h libxslt/xslt.h libxslt/xsltInternals.h libxslt/xsltconfig.h libxslt/xsltutils.h limits.h linux/asequencer.h linux/awe_voice.h locale.h machine/endian.h machine/vmparam.h magick/magick.h main.h memory.h mntent.h nan.h net/errno.h netaccess.h netdb.h netinet/in.h netinet/tcp.h openssl/asn1.h openssl/bio.h openssl/evp.h openssl/pem.h openssl/pkcs12.h openssl/pkcs7.h openssl/rand.h openssl/ssl.h openssl/x509.h openssl/x509_vfy.h openssl/x509v3.h osreldate.h paths.h pcreposix.h pthread.h pwd.h regex.h resolv.h sched.h scheduler.h setjmp.h setproctitle.h signal.h socket.h soundcard.h string.h strings.h stropts.h sys/acl.h sys/asoundlib.h sys/audio.h sys/audioio.h sys/conf.h sys/dir.h sys/exec.h sys/file.h sys/filio.h sys/fs/s5param.h sys/fs_types.h sys/immu.h sys/ioctl.h sys/ipc.h sys/machine.h sys/mman.h sys/mntctl.h sys/mnttab.h sys/mount.h sys/param.h sys/pstat.h sys/resource.h sys/select.h sys/shm.h sys/signal.h sys/socket.h sys/soundcard.h sys/stat.h sys/stream.h sys/stropts.h sys/sysmips.h sys/sysnews.h sys/time.h sys/timeb.h sys/times.h sys/types.h sys/ucred.h sys/uio.h sys/un.h sys/user.h sys/utsname.h sys/vfs.h sys/vmount.h sys/wait.h sysent.h syslog.h termios.h test.h tiffio.h time.h uio.h un.h unistd.h utime.h varargs.h volmgt.h windows.h xslt.h zlib.h