xfce4-taskmanager 1.1.0 does not require libxfcegui4-devel anymore. Therefore the Buildrequirement can (and imho should) be removed. Furthermore there is an optional condition for providing libgksu2 support. http://pkgs.fedoraproject.org/cgit/xfce4-taskmanager.git/plain/xfce4-taskmanager.spec
libxfcegui4-devel has received deprecation mark (or something like this) a couple of days ago. The requirements for libxfcegui4-devel is still within the xfce4-taskmanager.spec file and thus now causes trouble. Rather than building from the spec, the buildprocess simply aborts with an error. This should clearly be fixed.
https://koji.fedoraproject.org/koji/taskinfo?taskID=9647563 https://koji.fedoraproject.org/koji/taskinfo?taskID=9647568