First, SGI never guarantees that an executable built on a later IRIX release will work on an earlier release. Sometimes it works; more often than not it does not. GLUT takes advantage of a new X optimization in IRIX 6.3 called "fast atoms". This optimization lets X clients determine common atom values without an X server round-trip. This helps X performance.
If you compile the GLUT library on an IRIX 6.3 or IRIX 6.4 machine, the library will support fast atoms. This will mean that if you run executables linked against the "fast atom enabled" version of the GLUT library, you'll get a run-time link error saying something like:
17062:glut_example: rld: Fatal Error: attemped access to unresolvable symbol in projtex: _XSGIFastInternAtom
Do not be alarmed. If you want, you can recompile the GLUT library with the -DNO_FAST_ATOMS and get a version of the library that doesn't have the support so that GLUT executables built with a library compiled without "fast atoms" can work on earlier IRIX releases. Note that even if you do compile with -DNO_FAST_ATOMS, there is still no guarantee that an IRIX executable compiled on a newer release will actually work on an older release (but at least you'll have a chance!).