Fix for installing old gems on modern linuxes (Ubuntu 26.04)

On Ubuntu 26.04, installing gems with native extensions regularly fails with various compiler warnings. This is usually due to your updated C or C++ compiler enforcing different standards.

While it is possible to work around most cases with various flags, the simplest solution seems to be:

• Install the older C and C++ compilers using

  apt install gcc-13 g++-13
  

• Install gems with

  MAKEFLAGS="CC=gcc-13 CXX=g++-13" bundle install