Create the glog include directories and library directories
only if we've already checked that the user is using
glog (and not miniglog).

Otherwise, @GLOG_INCLUDE_DIR@ and @GLOG_LIBRARY@ will be
empty and the generated files will cause Cmake errors.

Change-Id: Ia0ce9cf98f7d56c83b2285f72f45b8e0d5dc34c4