Grepping the xfdesktop sources, I see that the configure.ac has a libSM check. However, I don't see any libSM reference anywhere else in the code, and the final executable seems not to link to it. Therefore, I think this is a leftover check.
Yeah, let's let libxfce4ui determine what it needs to provide session management. Dropped the requirement in: commit 57a3e70ed8c958d4a572f78aeefc77e8c9b1f266 Author: Eric Koegel <eric.koegel@gmail.com> Date: Thu Mar 26 14:49:53 2015 +0300 Drop explicit LibSM check (Bug #11744) We get session management handling from libxfce4ui so let that bring in the proper dependency. http://git.xfce.org/xfce/xfdesktop/commit/?id=57a3e70ed8c958d4a572f78aeefc77e8c9b1f266