Adding librt to link libraries for SuiteSparse_config on Linux.

- SuiteSparse_config (SuiteSparse >= 4.0) requires librt if compiled
  with timing support, which is the default on Linux & Unix (except
  OSX).
- This was previously raised as issue 124:
  https://code.google.com/p/ceres-solver/issues/detail?id=124.

Change-Id: I08ada41cd28d2e63a4bf342e143e23fe8213e9ee