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Edit: Adding example of looking for dependencies conditionally in CMake

First let me explain the find_package(catkin REQUIRED COMPONENTS ...) mechanism.

This:

find_package(catkin REQUIRED COMPONENTS foo bar baz)

message("Libraries: ${catkin_LIBRARIES}")

Is the same as:

find_package(foo REQUIRED)
find_package(bar REQUIRED)
find_package(baz REQUIRED)

message("Libraries: ${foo_LIBRARIES};${bar_LIBRARIES};${baz_LIBRARIES}")

So any variable which starts with catkin_ will be the aggregate of the items you passed as COMPONENTS. Therefore, you can find package things one at a time and act accordingly:

find_package(catkin REQUIRED COMPONENTS required_dep1 required_dep2)

set(MY_SRCS src/main.cpp ...)
set(MY_LINK_INCLUDE_DIRS ${catkin_INCLUDE_DIRS})
set(MY_LINK_LIBRARIES ${catkin_LIBRARIES})

find_package(optional_dep1)

if(optional_dep1_FOUND)
    list(APPEND MY_SRCS src/optional_dep1_code.cpp)
    list(APPEND MY_LINK_INCLUDE_DIRS ${optional_dep1_INCLUDE_DIRS})
    list(APPEND MY_LINK_LIBRARIES ${optional_dep1_LIBRARIES})
    # Anything else you need to do to enable use of the optional dep, like add definitions
    add_definitions("-DUSE_OPTIONAL_DEP1=1")
endif()

include_directories(include ${MY_LINK_INCLUDE_DIRS})

catkin_package()

add_executable(my_exec ${MY_SRCS})
target_link_libraries(my_exec ${MY_LINK_LIBRARIES})

You can imagine how you could conditionally do just about anything based on the result of optional_dep1_FOUND.