Add note about glog not currently compiling against gflags 2.1.
Add note about glog not currently compiling against gflags 2.1.

- Also fix a typo in pointer to EIGENSPARSE documentation.

Change-Id: I1448fcf3680a65362c6f9b2d8a5ab0259da92364

DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,ysonggit/ceres-solver,entylop/ceres-solver,whuaegeanse/ceres-solver,ysonggit/ceres-solver,ysonggit/ceres-solver,entylop/ceres-solver,ysonggit/ceres-solver,entylop/ceres-solver,whuaegeanse/ceres-solver,whuaegeanse/ceres-solver,DougFirErickson/ceres-solver,entylop/ceres-solver,whuaegeanse/ceres-solver