Fix namespace issues and CMake configuration.

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.