[cmake] Catch non-existing target properties for the dependency lookout.

[jnbrunet]

Not sure why it happen, but it was reported by @pedroperrusi that some targets might not have the LIBRARY_OUTPUT_DIRECTORY property, hence making the target dependencies lookout for RPATHs fail:

CMake Error at CMake/SofaPython3Tools.cmake:158 (file):
  file RELATIVE_PATH must be passed a full path to the file:
  DEPENDENCY_LIBRARY_OUTPUT_DIRECTORY-NOTFOUND
Call Stack (most recent call first):
  bindings/SofaRuntime/CMakeLists.txt:22 (SP3_add_python_module)


Python module 'SofaRuntime' added to site-packages/SofaRuntime
CMake Error at CMake/SofaPython3Tools.cmake:158 (file):
  file RELATIVE_PATH must be passed a full path to the file:
  DEPENDENCY_LIBRARY_OUTPUT_DIRECTORY-NOTFOUND
Call Stack (most recent call first):
  bindings/SofaGui/CMakeLists.txt:18 (SP3_add_python_module)

This PR will make sure that the dependency lookout don't fail in such a case.

[guparan]

if (DEPENDENCY_LIBRARY_OUTPUT_DIRECTORY) will do this double check 😉

[jnbrunet]

Thanks @guparan , it should be good now!