Ignore warnings from within Eigen/SparseQR (3.2.2).

- As reported by Chris Sweeney on the mailing list, Eigen 3.2.2
  trips various warnings in Eigen/SparseQR when compiling with GCC.
- Following Petter Strandmark's suggestion, Eigen headers are now
  treated as system headers, which implicitly suppresses all warnings.

Change-Id: I104e8cb3f00935cefb894089bea827771e0e9fd0