playbook: Install gcc 4.9 on Debian 9

As standalone-shared is for ERP5-related mainly, and those do require old gcc version, install (in hackich way) gcc-4.9 on Debian 9.

Also gcc and g++ (and others) are set to be default as 4.9.

The hackish way is based on https://unix.stackexchange.com/questions/334888/how-install-g-4-9-on-debian-stretch