This is a known problem with flex and kdesdk. See the following thread on the kde-freebsd mailing list:
http://mail.kde.org/pipermail/kde-fr...er/003894.html
You can try to apply the following patch to the textproc/flex port:
http://mail.kde.org/pipermail/kde-fr...ttachment.diff
Code:
# cd /usr/ports/textproc/flex
# patch </path/to/patch
# pkg_delete -fx flex
# make install clean
Alternatively, just build KDE without kdesdk.