Downgrade GCC from 5.2.1 to 4.9 Ubuntu 15.10

Like @steeldriver suggests, you need to use update-alternatives. Exactly step 3 on this question. Since at this point 4.9 is 4.9.3, which is not supported, you'll need 4.8.

sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.8 40 --slave /usr/bin/g++ g++ /usr/bin/g++-4.8

sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-5 50 --slave /usr/bin/g++ g++ /usr/bin/g++-5

Remember to choose gcc-4.8 before running the installer:

sudo update-alternatives --config gcc

and after, to switch back to having gcc-5 by default.

(Since you're doing it to install the CUDA SDK, I'll add that if you want to test the samples, you may want to read this.)