Fix the broken build.

1. When protocol buffers support is enabled CompressedRowSparseMatrix
   has a missing virtual method.
2. When SuiteSparse is missing, covariance_test tries to run the
   large scale covariance computation test.

Thanks to Alex Stewart for reporting #1.

Change-Id: I4238c966036362175e31749595ea8bb6f12a696c
2 files changed