Environment variables - where are they stored by linux, how do I change them and is it safe to do so?

I am trying to configure a build from source an old version of gcc on my dev machine. The documentation highlight several environment variables that need to be changed in order to do this. I was wondering how to do this and also where linux stores them and if a system restart is required for any changes to be registered??

The Global environment variables of your system are stored in /etc/environment.
Any changes here will get reflected throughout the system and will affect all users of the system. Also, you need a Reboot, for any changes made here to take effect.

User level Environment variables are mostly stored in .bashrc and .profile files in your Home folder. Changes here only affect that particular user. Just close and open the terminal for configuration changes to take place.

Edit : If you don't want to Reboot or restart your terminal, you can make use of the source command.
Eg. source /etc/environment or source .bashrc

What you need is local temporal changes. You are not going to need them tomorrow, when you are doing something else, it may break the thing you do tomorrow. However you could save it as a build-script/makefile for use tomorrow.

export VARIABLE=value
export VARIABLE2=another_value
gcc …

or

VARIABLE=value VARIABLE2=another_value gcc …

note: If using make, the first example wont work. Make starts a new shell (sh) for each line.

You should place your environment variables on a per-user basis in ~/.pam_environment

Create the file if it doesn't exist. See here