To enable NPTL on Gentoo I think you should, edit /etc/make.conf, adding nptl to your USE variable.
For RHEL and Fedora Core.
Native POSIX Thread Library (NPTL) settings
Native POSIX Thread Library (NPTL), a new implementation of POSIX threads for Linux, provides performance improvements and increased scalability.
To check NPTL to verify that it is on, you can use the following command:
See also http://en.wikipedia.org/wiki/NPTL