[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

CMakeLists.txt

@@ -83,7 +83,7 @@ endif ()
 search_for_boost()
 # Optional dependencies
 add_optional_dependency("octomap >= 1.6")
-if (OCTOMAP_INCLUDE_DIRS AND OCTOMAP_LIBRARY_DIRS)
+if (OCTOMAP_FOUND)
   include_directories(SYSTEM ${OCTOMAP_INCLUDE_DIRS})
   link_directories(${OCTOMAP_LIBRARY_DIRS})
   SET(HPP_FCL_HAVE_OCTOMAP TRUE)