Cmake判断使用lib64还是lib32

# won't work before project()!   
#https://stackoverflow.com/questions/4845198/fatal-error-no-target-architecture-in-visual-studio 
if(CMAKE_SIZEOF_VOID_P EQUAL 8)
    message("64 bit")
elseif(CMAKE_SIZEOF_VOID_P EQUAL 4)
    message("32 bit")
endif()