Update Wukong+G docs and fix compile errors

gpu_hash.cu also depends on global.hpp, which causes "multiple definition" link errors. Because .cu file likes .cpp file, hence gpu_hash.cu will be compiled into gpu_hash.o, and will be linked with wukong.o to generate the executable file.
So I make the symbols in global.hpp as weak symbols, since compiler will randomly choose one definition for weak symbols and avoiding "multiple definitions" errors.