Merge remote-tracking branch 'google/release'

Release version 2.1.0.Build gflags using CMake, packaging using CPack.

Release version 2.0.

Release version 1.7.

Release version 1.6.

Release version 1.5.

Release version 1.4.

Release version 1.3.

Release version 1.2.

Release version 1.1.