先清除原来的版本：
sudo update-alternatives --remove-all gcc
sudo update-alternatives --remove-all g++
安装指定版本
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.8 48 \
--slave /usr/bin/gcc-ar gcc-ar /usr/bin/gcc-ar-4.8 \
--slave /usr/bin/gcc-nm gcc-nm /usr/bin/gcc-nm-4.8 \
--slave /usr/bin/gcc-ranlib gcc-ranlib /usr/bin/gcc-ranlib-4.8
配置更改：
sudo update-alternatives --config gcc
sudo update-alternatives --config g++
查看修改完的版本：
gcc --version
gcc (Ubuntu/Linaro X.X.X-1ubuntu1~14.04) X.X.X
Copyright (C) 20XX Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
g++ --version
g++ (Ubuntu/Linaro X.X.X-1ubuntu1~14.04) X.X.X
Copyright (C) 20XX Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.