Use target qualifier for checking the cpp typesupport exists (#238)

Signed-off-by: Ryan Friedman <[email protected]>

Author: Ryanf55

sensor_msgs/CMakeLists.txt

@@ -64,7 +64,7 @@ if(BUILD_TESTING)
   ament_lint_auto_find_test_dependencies()
 endif()
 
-if(cpp_typesupport_target)
+if(TARGET "${cpp_typesupport_target}")
   add_library(${PROJECT_NAME}_library INTERFACE)
   target_include_directories(${PROJECT_NAME}_library INTERFACE
   "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>"