Add proper header only project so that it's properly detected by IDEs.

doc/source/snipsets/CMakeLists.txt

@@ -35,6 +35,7 @@ add_custom_target(build-images
   DEPENDS "${DOCUMENTATION_IMAGES}")
 
 link_libraries(${Boost_PROGRAM_OPTIONS_LIBRARY_RELEASE} ${FreeImage_LIBRARY})
+link_libraries(Mln)
 
 add_executable(erosion-cli erosion-cli.cpp)
 add_executable(staff_lines staff_lines.cpp)