Fix CMake unused variable warnings

Introduced by commit 8c9868f5, the variables "ExternalProjectsContrib_DIR"
and "PluginsContrib_DIR" where unconditionally passed down when
configuring CTK inner project.

This commit avoid the "unused variable" warnings by checking the value
of "CTK_USE_CONTRIBUTED_PLUGINS" variable before passing the variables.
Merge pull request #345 from jcfr/fix-unused-variable-warnings

Fix CMake unused variable warnings