cmakelists with boost not found

[xiefangping]

In your cmakelists, even if boost not found, still unittest and sandbox will be built, which cause a problem.

# Boost serialization for performance sandbox
find_package(Boost COMPONENTS serialization)

if(Boost_FOUND)
  include_directories(SYSTEM ${Boost_INCLUDE_DIRS})
endif(Boost_FOUND)
  
enable_testing()
add_subdirectory(unittests)

add_subdirectory(sandbox)

[AzothAmmo]

The unit tests do not use boost, and the portions of sandbox that require boost are already guarded: https://github.com/USCiLab/cereal/blob/develop/sandbox/CMakeLists.txt

[AzothAmmo]

This should be fixed now in the sense that the boost unit test (variadic) is now behind Boost_FOUND. See #568