Fix detection of octomap using pkgconfig
Fix detection of octomap using pkgconfig

If octomap is installed on the system library search path (e.g. from a
distribution package manager,) pkg_check_modules may not necessarially
set the OCTOMAP_LIBRARY_DIRS variable. In that case, the check for
OCTOMAP_LIBRARY_DIRS may fail even though the package was found,
preventing fcl from building against octomap. This commit checks the
pkg_check_modules _FOUND variable first, and then checks for the include
and library dirs if octomap was not found by pkgconfig.

mamoll/fcl,hsu/fcl,hsu/fcl,hsu/fcl,mamoll/fcl,mamoll/fcl