Hello.

I have 2 compilers installed on a shell, 3.3.6 and 4.2. I have a certain source code that needs to be compiled with the former.

How do I do that?

There's a handy tool called gcc-config. You might need to install it. Run this will show you a list of installed gcc's. Then
This will change gcc's. Then run the same commands to revert back.

I've checked and it is not installed, and I can't seem to find a download for it. Do you have a link? and will it work without having root access?

Instead of running "make" run "make CC=gcc-3.3.6 CXX=gcc-3.3.6-g++".
CC variable specifies C compiler executable, CXX specifies C++ compiler executable. In C only projects you don't have to specify "CXX=..." part. But make sure that on your system gcc 3.3.6 executable is named gcc-3.3.6 (and g++ for 3.3.6 is named gcc-3.3.6-g++) and not something else.

I get the following error:

you can also just make new symlinks for gcc, g++ and cpp to the versions you want to use.

ln -sf /usr/bin/gcc-3.3.6 /usr/bin/gcc

and so on.

I think, I can guess, what is going on : May be, you used
2 Slackware packages to have 2 compilers.
That is simply not possible with 3.3.x / 4.x, too many conflicts.

The solution is to remove the "extra compiler" with # 'removepkg'
Next : Reinstall the system compiler to insure, that everything
is in place.

To make an extra usable compiler package, e.g. gcc-4.x :
Unpack gcc-4.x--.tgz in a new directory in /home .
Delete conflicting files, rename files in usr/bin :
1) delete install/doinst.sh
2) delete usr/lib/libgcc_s.so.1
3) in usr/bin : mv cpp cpp-4.1 ( or 4.2 if that's the version )
4) in usr/bin : mv gcov gcov-4.1 ( 4.2..)
and delete usr/bin/gccbug.
5) delete usr/share usr/doc usr/include usr/man usr/info
and usr/lib/classpath .

Use # 'makepkg' to repack ( use a new package name )
Say yes to 'chmod 0755'

If the above is a compiler, capable of working in that system,
it can now do it with no conflicts, and without overwriting
"system compiler files", when installed.
....
Good luck !
....

You can use the "-V 3.3.6" argument it gcc version 3.36 is installed. You may just have a gcc33 package. I don't know if that would be version 3.3.6.