I am using glibc 2.3.4-20040000-r1. The built_with_use check against glibc in the neon ebuild doesn't account for old versions of glibc that do not have nptl or nptlonly in IUSE. Reproducible: Always Steps to Reproduce: emerge neon (on system with old glibc) The easy fix seems to be adding "--missing false" to each built_with_use check for nptl and nptlonly
Nothing to fix here, upgrade your toolchain first.