nvidia-legacy-driver checks if glibc was built with nptl useflag. Since glibc-2.6 doesn't have that useflag, portage complains. I commented out the check, and my nvidia card works fine with glibc-2.6. The solution would be to check only for nptl if <glibc-2.6 Reproducible: Always
Dupe of bug #186449.
*** This bug has been marked as a duplicate of bug 186449 ***