Manage different GCC versions.

http://manpages.ubuntu.com/manpages/trusty/man8/update-alternatives.8.html

$ update-alternatives --list gcc
/opt/gcc-4.9/bin/gcc
/usr/bin/gcc-5
/usr/bin/gcc-7

$ sudo apt-get install gcc-9 g++9

$ which gcc-9
/usr/bin/gcc-9

$  which gcc
/usr/bin/gcc
$ gcc --version
gcc (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0
Copyright (C) 2017 Free Software Foundation, Inc.

$ sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-9 20
$ sudo update-alternatives --set gcc /usr/bin/gcc-9


$ sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-9 20
$ sudo update-alternatives --set g++ /usr/bin/g++-9

$ gcc --version
gcc (Ubuntu 9.2.1-17ubuntu1~18.04.1) 9.2.1 20191102