Fixes an FTBFS on openSuse

openSuse uses --no-undefined in CMAKE_EXE_LINKER_FLAGS,
CMAKE_MODULE_LINKER_FLAGS and CMAKE_MODULE_LINKER_FLAGS.

Just removed the undefined references.
This was first reported and fix was proposed by: Shawn W Dunn
(https://github.com/sfalken). This patch is based on his.

Closes lxde#288 lxde/lxqt-panel#90 and lxde/lxqt-panel#98