Linux-headers

The sys-kernel/linux-headers package contains the Linux system headers. These are part of the Linux kernel, however they are shipped separately (Linus explains why). The headers are used for packages like glibc.

A common question regarding linux-headers is whether you need to keep them synced with your kernel version. The answer is no. You can have a newer linux-headers version then your running kernel binary.

So for example, if you have kernel 4.1 installed, you can have linux-headers 4.4. If you compile your glibc with this newer headers and later upgrade to kernel 4.4, you will be able to use the new features of this kernel without recompiling glibc again.

For more information read glibc faq and this.