This way it is now possible to use gflags >= 2.1, where all the
functions were moved from google to gflags namespace.
This isn't currently used in blender, but for standalone repository
this change is essential.
This commit makes it possible to use Glog library for the debug logging.
For now only possible when using CMake and in order to use the logging
the WITH_CYCLES_LOGGING configuration variable is to be enabled.
When this option is not enabled or when using Scons there's no difference
in Cycles behavior at all, when using logging and no output to the console
impact is gonna to be minimal.
This is done in order to make it possible to have debug logging persistent
in code (without need to add it when troubleshooting some bug and removing
it afterwards).
For now actual logging is not placed yet, only all the functions needed for
the logging are written and so.
