Ensure local config.h is used if Ceres is already installed.

- Previously, if Ceres was already installed to the same location as one
  of the depenedencies (e.g. /usr/local) then as the BUILD_DIR/config
  path was necessarily added to the include_directories() list *after*
  the INCLUDE_DIRS for the dependencies, the config.h for the
  *installed* version of Ceres would be used instead of the locally
  configured version, as it would appear first on the path.
- Forcing the location of the locally configured config.h to the front
  of the include_directories() list ensures that it is found first
  (and is thus used) even if an installed version of Ceres also exists
  on the path.

Change-Id: I7b4f97a3ad0120bd11551f9b8eb6bffc66966eb4