I study EOS have a peroid of time, At begining use the version of 2018-02-14, everything is well. several days ago, update to v3.0.0, and encounter a lot of  errors,  almost break me down. Now make a note,  hope can give a little help to these who meet the same problem, and EOS can settle this bug as soon.
    1) build
     git clone https://github.com/eosio/eos --recursive
     cd eos
     ./eosio_build.sh
     there, meet the first error. install Mongodb timeout.  this error is because I use ubuntu virtual machine,  and set nat can be resolved by  set the virtual machine Network Address Translation(nat), then it can build success.
    2)  I build smart contract hello which completely same as officail given. It occur "fatal error  eosiolib/eosio.hpp" .   I change the include mode from "<eosiolib/eosio>"  to  ""eosio""  it works,.
    3) when update smart contract, "account.code_version != code_id: contract is already running this version of code"
    it because I only change the abi file, add no  function code print to cpp file, regenerate wast abi file,everything ok.