Replace include_directories() with target_include_directories().

- Now that we require CMake >= 3.x, we can roll the definition of the
  include directories for clients linking against Ceres into the
  exported Ceres CMake target via target_include_directories().
- This removes the requirement for CERES_INCLUDE_DIRS.

Change-Id: Ibe3bbf796339871d66138fc9520053d1766f09a8