Minimum Linux kernel version for glibc 2.17

[David Miller]

From: Mike Frysinger <vapier@gentoo.org>
Date: Wed, 9 May 2012 09:44:29 -0400

> i would prefer to keep the version checking in the features header accurate.  
> seems like the cost is already sunk, and removing them doesn't really gain 
> anything ?  we use this header verbatim in uClibc, so it'd be nice to not have 
> to fork it ...

I absolutely disagree, it is only confusing to keep around tests that
are no longer necessary.