Actually, it's GLE that fails to build. mesa-glu installs correctly, but GLE claims that it can't find libglut.
Fixed. emerge media-libs/glut first, then try xscreensaver again.
I did an 'emerge media-libs/glut' and then 'emerge media-libs/gle' (since that's the library that was failing before) and it still fails, with the same error about being unable to find the required glut library. This is on a vanilla r14 install.
Fix for this seems to be: edit your copy of the glut .ebuild, and change the $MESA_VER to be the same version as the Mesa you are using. Then remerge glut. Otherwise unmerge mesa, merge xfree-4.2.0-r5, and just merge glut. Do not merge Mesa again (check for emerge --world update). It should work fine with xfree's libGLU on most cards.
Try using Mesa-3.42 and let me know the results.
It's working great now. I apologize for not coming back and closing the ticket. My current build is of XFree-4.2.0-r2 and mesa-glu-3.5. Gle builds great.