Release version 2.1.0.Build gflags using CMake, packaging using CPack.
Merge remote-tracking branch 'google/release' into 'master'. Conflicts: gflags-vs2003.sln gflags-vs2010.sln gflags-vs2012.sln vsprojects/gflags_unittest/gflags_unittest-vs2010.vcxproj vsprojects/libgflags/libgflags-vs2010.vcxproj