Use Ceres_[SOURCE/BINARY]_DIR not CMAKE_XXX_DIR to support nesting.

- Using Ceres_[SOURCE/BINARY]_DIR (which are defined by CMake when
  project(Ceres) is called, in favour of CMAKE_[SOURCE/BINARY]_DIR
  enables Ceres to be nested within (and built by) a larger CMake
  project (which also contains other projects).
- CMAKE_[SOURCE/BINARY]_DIR always refers to the top-level source
  and binary directories (i.e. the first encountered), as a result if
  Ceres is a nested project within a larger project, these would not
  correctly identify the source/binary directories for Ceres (as they
  would refer to the root project in which Ceres is nested).
- Using Ceres_[SOURCE/BINARY]_DIR should ensure that Ceres always uses
  the correct source/binary directories, irrespective of whether Ceres
  is nested or not.

Change-Id: I62226ea3f6552b1d7e2bdac1aef02f1f489ae55e