Use system recastnavigation instead of bundled one

diff --git a/CMakeLists.txt b/CMakeLists.txt
index d69352c94..cfb53dc5b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -596,7 +596,16 @@ endif(WIN32)
 # Extern
 set(RECASTNAVIGATION_STATIC ON CACHE BOOL "Build recastnavigation static libraries")
 
-add_subdirectory (extern/recastnavigation EXCLUDE_FROM_ALL)
+add_library(RecastNavigation::DebugUtils SHARED IMPORTED)
+add_library(RecastNavigation::Detour SHARED IMPORTED)
+add_library(RecastNavigation::Recast SHARED IMPORTED)
+set_target_properties(RecastNavigation::DebugUtils PROPERTIES
+	IMPORTED_LOCATION GENTOO_RECAST_LIBDIR/libDebugUtils.so)
+set_target_properties(RecastNavigation::Detour PROPERTIES
+	IMPORTED_LOCATION GENTOO_RECAST_LIBDIR/libDetour.so)
+set_target_properties(RecastNavigation::Recast PROPERTIES
+	IMPORTED_LOCATION GENTOO_RECAST_LIBDIR/libRecast.so)
+
 add_subdirectory (extern/osg-ffmpeg-videoplayer)
 add_subdirectory (extern/oics)
 if (BUILD_OPENCS)