Fix install path for CeresConfig.cmake to be architecture-aware.

- Previously we were auto-detecting a "64" suffix for the install path
  for the Ceres library on non-Debian/Arch Linux distributions, but
  we were installing CeresConfig.cmake to an architecture independent
  location.
- We now install CeresConfig.cmake to lib${LIB_SUFFIX}/cmake/Ceres.
- Also make LIB_SUFFIX visible to the user in the CMake GUI s/t they can
  easily override the auto-detected value if desired.
- Reported by jpgr87@gmail.com as Issue #194.

Change-Id: If126260d7af685779487c01220ae178ac31f7aea