centos：安装gcc g++ 6以上

ref:https://blog.youkuaiyun.com/zhizhengguan/article/details/109720174

安装g++7以上
默认yum安装的是4.8.5，如果想要g++7以上：

On Ubuntu try the following:

  sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
  sudo apt-get update -y
  sudo apt-get install g++-7 -y

On CentOS try the following:

  sudo yum install centos-release-scl
  sudo yum install devtoolset-7
  scl enable devtoolset-7 bash