To install the compiler, you need five files from Slackware 3.5:   binutils, egcs, gmake, libc and linuxinc.   You will find these files in /slakware/d1.   You should use the pkg command to install the files:
----------------
pkg binutils.tgz
pkg egcs.tgz
pkg gmake.tgz
pkg libc.tgz
pkg linuxinc.tgz
----------------
This will take about 15mb of HD space

You may need to install additional packages from /slakware/d1 if your source code needs them.   For example, if compilation stops because bison can not be found, you should install bison.tgz.   Or, if you are told that curses.h is missing, you should install ncurses.tgz.