How to correctly include navfn/navfn_ros.h ?

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Hello , I have been trying to get the exploration stack working on hydro ( catkin ) and for the most part it works, but whenever I try to include the navfn/navfn_ros.h in my header file as show on wiki.ros.org/navfn, and run catkin_make, I get the following error :

 In file included from /home/vicos/explore_ws/src/explorer/src/FrontierExplorer.cpp:1:0:
/home/vicos/explore_ws/src/explorer/include/explorer/FrontierExplorer.h:19:29: fatal error: navfn/navfn_ros.h: No such file or directory
compilation terminated.

Here is a part of my header file :

#ifndef FRONTIER_EXPLORATION_H
#define FRONTIER_EXPLORATION_H

#include <ros/ros.h>
#include <visualization_msgs/Marker.h>
#include <boost/bind.hpp>
#include <boost/thread.hpp>
#include <move_base_msgs/MoveBaseAction.h>
#include <actionlib/client/simple_action_client.h>
#include <vector>
#include <costmap_2d/costmap_2d_ros.h>
#include <costmap_2d/costmap_2d.h>
#include <geometry_msgs/PoseStamped.h>
#include <nav_msgs/GetMap.h>

#include <explorer/FrontierFinder.h>
#include <explorer/Visualizer.h>

#include <navfn/navfn_ros.h>
#include <navfn/navfn.h>

CMakeLists.txt :

cmake_minimum_required(VERSION 2.8.3)
project(explorer)

find_package(catkin REQUIRED COMPONENTS
  actionlib
  costmap_2d
  geometry_msgs
  move_base_msgs
  navfn
  nav_core
  roscpp
  visualization_msgs
  tf
)

 find_package(Boost REQUIRED COMPONENTS system)

catkin_package(
  INCLUDE_DIRS include
  LIBRARIES ${PROJECT_NAME} #explorer
  CATKIN_DEPENDS 
   actionlib 
   costmap_2d 
   geometry_msgs 
   move_base_msgs 
   navfn 
   roscpp 
   visualization_msgs 
   tf
)

include_directories(
    include
  ${catkin_INCLUDE_DIRS}
)

add_executable(FrontierExplorer src/FrontierExplorer.cpp src/FrontierFinder.cpp src/Visualizer.cpp)
target_link_libraries(FrontierExplorer ${catkin_LIBRARIES})

package.xml :

<?xml version="1.0"?>
<package>
  <name>explorer</name>
  <version>0.0.1</version>
  <description>The explorer package</description>
  <maintainer email="saprunov.luka@gmail.com">vicos</maintainer>
  <license>BSD</license>

  <buildtool_depend>catkin</buildtool_depend>

  <build_depend>actionlib</build_depend>
  <build_depend>costmap_2d</build_depend>
  <build_depend>geometry_msgs</build_depend>
  <build_depend>move_base_msgs</build_depend>
  <build_depend>navfn</build_depend>
  <build_depend>roscpp</build_depend>
  <build_depend>visualization_msgs</build_depend>
  <build_depend>tf</build_depend>

  <run_depend>actionlib</run_depend>
  <run_depend>costmap_2d</run_depend>
  <run_depend>geometry_msgs</run_depend>
  <run_depend>move_base_msgs</run_depend>
  <run_depend>navfn</run_depend>
  <run_depend>roscpp</run_depend>
  <run_depend>visualization_msgs</run_depend>
  <run_depend>tf</run_depend>
</package>

running rospack find navfn returns

/opt/ros/hydro/share/navfn

and if I run rospack depends explorer (my modified exploration stack)

genmsg
gencpp
genlisp
genpy
message_generation
catkin
console_bridge
cpp_common
rostime
roscpp_traits
roscpp_serialization
message_runtime
std_msgs
actionlib_msgs
rosbuild
rosconsole
rosgraph_msgs
xmlrpcpp
roscpp
rosgraph
rospack
roslib
rospy
rosclean
rosmaster
rosout
rosparam
roslaunch
rosunit
rostest
actionlib
rosbag_storage
topic_tools
rosbag
rosmsg
rosservice
dynamic_reconfigure
geometry_msgs
sensor_msgs
message_filters
rostopic
rosnode
roswtf
tf2_msgs
tf2
tf2_py
tf2_ros
tf
angles
laser_geometry
nav_msgs
map_msgs
pcl_msgs
pcl_conversions
bond
smclib
bondcpp
class_loader
pluginlib
nodelet
nodelet_topic_tools
pcl_ros
visualization_msgs
voxel_grid
costmap_2d
move_base_msgs
nav_core
navfn

both of which suggest that the navfn is present and correctly configured on the system and the move_base node ( which seems to use navfn ) works perfectly. So, why do I get the error ?

Thank you in advance for answering my question / helping with my problem !