Fix CMake warning for rt and pthread.

[mikepurvis]

Squelches the following CMake warning when building on Linux:

-- ==> add_subdirectory(serial)
CMake Warning at share/catkin/cmake/catkin_package.cmake:166 (message):
  catkin_package() DEPENDS on 'rt' but neither 'rt_INCLUDE_DIRS' nor
  'rt_LIBRARIES' is defined.
Call Stack (most recent call first):
  share/catkin/cmake/catkin_package.cmake:102 (_catkin_package)
  serial/CMakeLists.txt:14 (catkin_package)

CMake Warning at share/catkin/cmake/catkin_package.cmake:166 (message):
  catkin_package() DEPENDS on 'pthread' but neither 'pthread_INCLUDE_DIRS'
  nor 'pthread_LIBRARIES' is defined.
Call Stack (most recent call first):
  share/catkin/cmake/catkin_package.cmake:102 (_catkin_package)
  serial/CMakeLists.txt:14 (catkin_package)

However, this would also be fixed by merging #133, so it depends a bit on the next steps for that PR.

[wjwwood]

Since I'm probably going to decline or only use part of #133, this might still be relevant for the current version (which will remain the version in Kinetic).

[wjwwood]

@ros-pull-request-builder please test this