Fix CTK build when not all plugins/apps/libraries are enabled.

Following commit 95f4262 teaching the build system how to understand
the USE_SYSTEM option, the enabling variables were systematically set
in the ctkBlocCheckDependencies module.

Since the logic deciding if "find_package(PackageName)" should be called
was simply done by just checking if the
"< <PackageName>_enabling_variable>_FIND_PACKAGE_CMD" variable was set
or not, the build was failing when done without all external project
required.

See #418
Merge branch '418-support-use-system-option'

* 418-support-use-system-option:
  Fix CTK build when not all plugins/apps/libraries are enabled.