Added guards for gnu_source definition in CMake.
Added guards for gnu_source definition in CMake.

cramja/incubator-quickstep,hbdeshmukh/incubator-quickstep,tarunbansal/incubator-quickstep,UWQuickstep/quickstep,zuyu/incubator-quickstep,zuyu/incubator-quickstep,UWQuickstep/quickstep,jianqiao/incubator-quickstep,tarunbansal/incubator-quickstep,zuyu/incubator-quickstep,hbdeshmukh/incubator-quickstep,hbdeshmukh/incubator-quickstep,tarunbansal/incubator-quickstep,hbdeshmukh/incubator-quickstep,jianqiao/incubator-quickstep,UWQuickstep/quickstep,UWQuickstep/quickstep,cramja/incubator-quickstep,cramja/incubator-quickstep,jianqiao/incubator-quickstep,jianqiao/incubator-quickstep,UWQuickstep/quickstep,cramja/incubator-quickstep,zuyu/incubator-quickstep,tarunbansal/incubator-quickstep