The Slackware 13.37 libical package is broken.
From a message I sent November 3rd 2011:
For kdepim, the build fails with today's SVN because of
broken Slackware libical package: kludging a supplemental
"-I/usr/include/libical" (I did it with the libical.pc
file but it can be made in the build script) makes kdepim build fine.
Oof! November --- so long ago. :)
Thank you!
Darrell