add a check that package.xml declared a build dependency on catkin_simple

[tfoote]

We do this in catkin and it saves a lot of easy mistakes. Catkin simple could do the same.

What catkin says.

CMake Error at /opt/ros/hydro/share/catkin/cmake/catkin_package.cmake:112 (message):
  catkin_package() 'catkin' must be listed as a buildtool dependency in the
  package.xml
Call Stack (most recent call first):
  /opt/ros/hydro/share/catkin/cmake/catkin_package.cmake:98 (_catkin_package)
  /home/tfoote/work/trailmix/devel/share/catkin_simple/cmake/catkin_simple-extras.cmake:170 (catkin_package)
  Redwood/3rdparty/gflags_catkin/CMakeLists.txt:34 (cs_export)

[wjwwood]

If you don't have a build dependency on catkin_simple, then catkin_simple isn't guaranteed to be there, so it will more likely fail on find_package(catkin_simple REQUIRED).

[NikolausDemmel]

I will try to add such a check in #31. Note that in that light solving ros/catkin#789 would be nice such that catkin_simple packages would not need to declare both buildtool-dependencies on catkin and catkin_simple. catkin_simple should be enough ideally.