- Previously we overwrote the default (empty) config.h in the source
  tree with a configured config.h, generated using the current compile
  options.
- This was undesirable as it could lead to inadvertant commits of the
  generated config.h.

- This patch moves the default config.h to <src>/config/ceres/internal,
  separate from the other headers, thus if Ceres is compiled without
  CMake this directory will now also have to be included.  This
  directory is _not_ added to the CMake include directories for Ceres
  (thus the default config.h is never used when compiling with CMake).
- When using CMake, the generated config.h is now placed in
  <build>/config/ceres/internal, which is in turn added to the include
  directories for Ceres when it is compiled, and the resulting config.h
  is copied to ceres/internal when installed.

Change-Id: Ib1ba45e66e383ade2ebb08603af9165c1df616f2