Hi,
Good job with stlviewer, thank you for sharing it!
Just a small fix needed:
It creates a string like this for example:
gcc -Wall -o stlviewer -D_OPENGL_ -D_Linux_ -lglut -lGL -lGLU -lm trackball.c stl.c stl_viewer.c

However, the switches must be at the end:
gcc -Wall -o stlviewer trackball.c stl.c stl_viewer.c -D_OPENGL_ -D_Linux_ -lGL -lGLU -lglut -lm