libc/{glibc,eglibc}: Don't download glibc-ports when not available

Don't download glibc-ports when glibc or eglibc version greater than 2.16,
because the "ports" source is mainline in the glibc or eglibc since version 2.17.

Signed-off-by: "Daniel Zimmermann" <netzimme@gmail.com>
Message-Id: <9c045ca3cf1b9dc89da3.1384602843@haus-VirtualBox>
Patchwork-Id: 291766
[yann.morin.1998@free.fr: slightly tweak subject, change variable name]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 # Bare metal config options

 ## select BARE_METAL

 ##

 ## help 'Bare metal' refers to those programs that run without any kernel.

 ## help

 ## help You probably want to say 'y' here if you plan to use your compiler

 ## help to build bootloaders. It is not yet suitable to build Linux kernels,

 ## help though, because the APCI stuff relies on the target C library headers

 ## help being available?!?!...