[CMake] use OCTOMAP_FOUND

As on some systems, we don't have OCTOMAP_INCLUDE_DIRS &
OCTOMAP_LIBRARY_DIRS but OCTOMAP_INCLUDEDIR & OCTOMAP_LIBDIR, and
therefore we get:

-- Checking for module 'octomap >= 1.6'
--   Found octomap , version 1.9.0
-- Pkg-config module octomap v1.9.0 has been detected with success.
-- FCL does not use Octomap